[SystemZ] Optimize 32-bit FPR<->GPR moves for z196 and above

Floats are stored in the high 32 bits of an FPR, and the only GPR<->FPR
transfers are full-register transfers.  This patch optimizes GPR<->FPR
float transfers when the high word of a GPR is directly accessible.
[SystemZ] Add test missing from r191764.