Recommit "[X86] Simplify some `pternlog` intrinsics" (2nd Try)

Original location of the unittests was Transforms/InstCombine/X86
which caused an issue as non-x86 targets ended up with a dependency on
X86 which isn't right.

Moved the unit tests to Target/X86 so should be good now.

Differential Revision: https://reviews.llvm.org/D145220