[SystemZ] Use STDY/STEY/LDY/LEY for VR32/VR64 in eliminateFrameIndex().

When e.g. a VR64 register is spilled to a stack slot requiring a long
(20-bit) displacement, it is possible to use an FP opcode if the allocated
phys reg allows it. This eliminates the use of a separate LAY instruction.

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D115406