[SystemZ] Don't shrink 64-bit FP constants.

Return false from ShouldShrinkFPConstant(), so that these constants are stored
in their full size on the constant pool, even if they could have been shrunk
and used with an extending load.

This is better since LD is faster than LDE, and it also enables reg/mem opcodes.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D117927