[SystemZ] Make sure not to call getZExtValue on a >64 bit constant.

Better use isZero() and isIntN() in SystemZTargetTransformInfo rather than
calling getZExtValue() since the immediate operand may be wider than 64 bits,
which is not allowed with getZExtValue().

Fixes https://bugs.llvm.org/show_bug.cgi?id=47600

Review: Simon Pilgrim