[TargetLowering] Fix undef vector element issue with true/false result handling

Fixed an issue with vector usage of TargetLowering::isConstTrueVal / TargetLowering::isConstFalseVal boolean result matching.

The comment said we shouldn't handle constant splat vectors with undef elements. But the the actual code was returning false if the build vector contained no undef elements....

This patch now ignores the number of undefs (getConstantSplatNode will return null if the build vector is all undefs).

The change has also unearthed a couple of missed opportunities in AVX512 comparison code that will need to be addressed.

Differential Revision: https://reviews.llvm.org/D26031

llvm-svn: 286238