[SystemZ] Use llvm::countl_zero and llvm::countr_zero (NFC)

isVectorConstantLegal calls findFirstSet and findLastSet, but we don't
rely on their ability to return std::numeric_limits<T>::max() on input
0.

This patch replaces those calls with calls to llvm::countl_zero and
llvm::countr_zero.

Due to an off-by-one error in the original code, the value of Upper
could change at bit N, where N is the index of the highest set bit in
SplatBitsZ, but the difference doesn't matter at the end.  Without
this patch, Upper could have bit N set.  With this patch, Upper never
has bit N set.  Either way, both calls to tryValue have this bit set
because the argument is ORed with SplatBitsZ.