[SystemZ] Avoid LER on z13 due to partial register dependencies

On the z13, it turns out to be more efficient to access a full
floating-point register than just the upper half (as done e.g.
by the LE and LER instructions).

Current code already takes this into account when loading from
memory by using the LDE instruction in place of LE.  However,
we still generate LER, which shows the same performance issues
as LE in certain circumstances.

This patch changes the back-end to emit LDR instead of LER to
implement FP32 register-to-register copies on z13.

llvm-svn: 263431