[X86] getTargetConstantBitsFromNode - remove unnecessary getZExtValue() (PR42486)

Don't use APInt::getZExtValue() if you can avoid it - eventually someone will call it with i128 or something that doesn't fit into 64-bits.

In this case it was completely superfluous as we'd moved the rest of the code to always use APInt.

Fixes the <1 x i128> addition bug in PR42486

llvm-svn: 364953