[AVX512] Don't create SHRUNKBLEND SDNodes for 512-bit vectors

There are no 512-bit blend instructions so we shouldn't create SHRUNKBLEND for them.

On a side note, it looks like there may be a missed opportunity for constant folding TESTM when LHS and RHS are equal.

This fixes PR34139.

Differential Revision: https://reviews.llvm.org/D36992

llvm-svn: 311572