This allows truncated splat / buildvector in isBoolConstant, to allow
certain not instructions to be recognized post-legalization, and allow
vselect to optimize.
An override for x86 avx512 predicated vectors is required to avoid an
infinite recursion from the code that detects zero vectors. From:
```
// Check if the first operand is all zeros and Cond type is vXi1.
// If this an avx512 target we can improve the use of zero masking by
// swapping the operands and inverting the condition.
```
0736f330b0