[SystemZ] Improve foldMemoryOperandImpl: vec->FP conversions

Use FP-mem instructions when folding reloads into single lane (W..) vector
instructions.

Only do this when all other operands of the instruction have already been
allocated to an FP (F0-F15) register.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D76705