Add peephole optimization to reduce MaskToVec and VecToMask conversions

[abhishekdahad-1]

On x64, specifically when supporting AVX512 there is an issue in the code gen which leads to extraneous MaskToVec and VecToMask conversions. This can be observed specifically on the System.Numerics.Tensors.IndexOfMax<Int32> benchmark.

The generated assembly will look like this:

This can be fixed by examining every VecToMask conversion and seeing if it fits this form:

MASK_2 = VecToMask(NOT(LCL_VAR))
and LCL_VAR is defined earlier in the program as the following:
LCL_VAR = MaskToVec(MASK_1)

Then we can reduce this expression to become:

MASK_2 = NOT(MASK_1)

The fixed assembly looks like this:

This fix causes a ~47 reduction for 128 BufferLength and ~55% reduction in runtime for 3079 BufferLength. The runtime on System.Numerics.Tensors.IndexOfMax<Int32> is compared below.

Before changes:

After changes:

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