[X86] Use llvm::countr_zero instead of findFirstSet (NFC)

At the call site of findFirstSet, ZMask | (1 << DstIdx) always have
exactly 3 bits set, and they are all among the 4 least significant
bits, so (ZMask | (1 << DstIdx)) ^ 15 has exactly one bit set.  Since
the argument to findFirstSet is nonzero, we can safely switch to
llvm::countr_zero.