[LivePhysRegs] Use .asMCReg() rather than cast (NFC)

[MacDue]

Should fix MSVC buildbots:

https://lab.llvm.org/buildbot/#/builders/197/builds/8321
https://lab.llvm.org/buildbot/#/builders/211/builds/1549

[llvm-ci]

LLVM Buildbot has detected a new failure on builder lldb-aarch64-windows running on linaro-armv8-windows-msvc-05 while building llvm at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/141/builds/11037

Here is the relevant piece of the build log for the reference

Step 6 (test) failure: build (failure)
...
UNSUPPORTED: lldb-api :: tools/lldb-dap/databreakpoint/TestDAP_setDataBreakpoints.py (1202 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/disassemble/TestDAP_disassemble.py (1203 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/disconnect/TestDAP_disconnect.py (1204 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/evaluate/TestDAP_evaluate.py (1205 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/exception/TestDAP_exception.py (1206 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/exception/cpp/TestDAP_exception_cpp.py (1207 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/exception/objc/TestDAP_exception_objc.py (1208 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/extendedStackTrace/TestDAP_extendedStackTrace.py (1209 of 2298)
UNSUPPORTED: lldb-api :: tools/lldb-dap/instruction-breakpoint/TestDAP_instruction_breakpoint.py (1210 of 2298)
UNRESOLVED: lldb-api :: tools/lldb-dap/io/TestDAP_io.py (1211 of 2298)
******************** TEST 'lldb-api :: tools/lldb-dap/io/TestDAP_io.py' FAILED ********************
Script:
--
C:/Users/tcwg/scoop/apps/python/current/python.exe C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/llvm-project/lldb\test\API\dotest.py -u CXXFLAGS -u CFLAGS --env LLVM_LIBS_DIR=C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./lib --env LLVM_INCLUDE_DIR=C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/include --env LLVM_TOOLS_DIR=C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./bin --arch aarch64 --build-dir C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/lldb-test-build.noindex --lldb-module-cache-dir C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/lldb-test-build.noindex/module-cache-lldb\lldb-api --clang-module-cache-dir C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/lldb-test-build.noindex/module-cache-clang\lldb-api --executable C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./bin/lldb.exe --compiler C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./bin/clang.exe --dsymutil C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./bin/dsymutil.exe --make C:/Users/tcwg/scoop/shims/make.exe --llvm-tools-dir C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./bin --lldb-obj-root C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/tools/lldb --lldb-libs-dir C:/Users/tcwg/llvm-worker/lldb-aarch64-windows/build/./lib --cmake-build-type Release --skip-category=watchpoint C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\llvm-project\lldb\test\API\tools\lldb-dap\io -p TestDAP_io.py
--
Exit Code: 1

Command Output (stdout):
--
lldb version 22.0.0git (https://github.com/llvm/llvm-project.git revision ee8c14be14deabace692ab51f5d5d432b0a83d58)
  clang revision ee8c14be14deabace692ab51f5d5d432b0a83d58
  llvm revision ee8c14be14deabace692ab51f5d5d432b0a83d58
Skipping the following test categories: ['watchpoint', 'libc++', 'libstdcxx', 'dwo', 'dsym', 'gmodules', 'debugserver', 'objc', 'fork', 'pexpect']


--
Command Output (stderr):
--
========= STDOUT =========



========= END =========

========= DEBUG ADAPTER PROTOCOL LOGS =========

1755948769.423700094 (stdio) received EOF



========= END =========

PASS: LLDB (C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\build\bin\clang.exe-aarch64) :: test_eof_immediately (TestDAP_io.TestDAP_io.test_eof_immediately)

FAIL: LLDB (C:\Users\tcwg\llvm-worker\lldb-aarch64-windows\build\bin\clang.exe-aarch64) :: test_incorrect_content_length (TestDAP_io.TestDAP_io.test_incorrect_content_length)

========= STDOUT =========