[SystemZ] Bugfix in SystemZVectorConstantInfo

In order to correctly load an all-ones FP NaN value into a floating point
register with a VGBM, the analyzed 32/64 FP bits must first be shifted left
(into element 0 of the vector register).

SystemZVectorConstantInfo has so far relied on element replication which has
bypassed the need to do this shift, but now it is clear that this must be
done in order to handle NaNs.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D89389