[WIP] Use SequenceEqual with Intrinsics to use both Vector sizes at once

[benaadams]

The .NET Vector type is always the largest supported size; so if Avx2 is enabled byte sequences have to be 32 or larger to benefit from vectorization.

Using the Intrinsics we can enable it using Sse2 for sizes 16 or larger and then switch to Avx2 for 32 or larger on Avx2 machines.

Also some tweaks for codegen compactness

The intrinsic variant is slightly better than the output the Jit generates as it drops a vmovaps instruction

G_M17140_IG11:
        C4A179100409         vmovupd  xmm0, xmmword ptr [rcx+r9]
        C4A179100C0A         vmovupd  xmm1, xmmword ptr [rdx+r9]
-       C5FC28D0             vmovaps  ymm2, ymm0
-       C5ED76D1             vpcmpeqd ymm2, ymm1
+       C5F974C1             vpcmpeqb xmm0, xmm0, xmm1
-       C5FDD7C2             vpmovmskb eax, ymm2
+       C579D7D0             vpmovmskb r10d, xmm0
-       83F8FF               cmp      eax, -1
+       4183FAFF             cmp      r10d, -1
-       0F858E000000         jne      G_M17103_IG14
+       752B                 jne      SHORT G_M17140_IG14

/cc @CarolEidt @fiigii @tannergooding

[benaadams]

Not quite sure where I've broken it atm

[jkotas]

Performance numbers to show that this actually makes a difference?

[benaadams]

Performance numbers to show that this actually makes a difference?

Currently regresses 😢

[benaadams]

Improves for large lengths > 512, but regresses for shorter lengths, which isn't what I was going for...

[jkotas]

Is the redundant !Avx2.IsSupported needed here? I would expect that the if (Avx2.IsSupported) above will be allow JIT to dead-code eliminate the else part.

[benaadams]

Tried lots of permutations nothing that's a significant and consistent win; will try another time.

Has given me ideas on the other vectorized methods where there should be easier wins.