Add handling for SIMD constants in Mono

[tannergooding]

Today mono has some SIMD support but it doesn't have any way to special case the introduction of "simd constants" and it always introduces "new values" by creating a "zero value" and then a separate insert node for each element.

This works and even gets optimized in most cases by LLVM. However, it also can represent a non-trivial amount of IR being introduced (Vector256<byte> is 32 inserts) and can hinder some opportunistic optimizations lighting up earlier.

I propose that mono introduce a MONO_EMIT_NEW_VCONST or similar. On LLVM this would defer down to llvm::ConstantDataVector (for the LLVM-C API this is LLVMConstVector).

Likewise it should take advantage of the other LLVM APIs such as LLVMConstExtractElement, LLVMConstInsertElement, and LLVMConstShuffleVector as this should lead to an increase in throughput and some better codegen.

[dotnet-issue-labeler]

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