[X86][AVX512] Add combine for TESTM

Add an X86 combine for TESTM when one of the operands is a BUILD_VECTOR(0,0,...).

TESTM op0, BUILD_VECTOR(0,0,...) -> BUILD_VECTOR(0,0,...)
TESTM BUILD_VECTOR(0,0,...), op1 -> BUILD_VECTOR(0,0,...)

Differential Revision:
https://reviews.llvm.org/D36536

llvm-svn: 310787