Refactor GenTreeJitIntrinsic to store the base type for SIMD operations as a var_types.

[anthonycanino]

This PR adjust GenTreeJitIntrinsic to save the base type of SIMD operation as a var_types and not CorInfoType. This is done to allow SIMD operations on types that are not part of the CIL.

Some notes on the changes:

1. simdBaseJitType has been renamed to simdBaseType and most flows that previous had both a simdBaseJitType and simdBaseType now only use the simdBaseType.
2. simdBaseType should now refer to the type returned via JitType2PreciseVarType.
3. Some instances required the less precise, JitType2VarType.
4. gtAuxiliaryJitType has been left unchanged. Some new functions have been introduced called getCorBaseTypeXX that keep the original behavior.

If we decide to also change gtAuxiliaryJitType I can make the changes and refactor things a bit more.

Current opening PR as draft to get a test run.

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