[SystemZ] Use LDE32 instead of LE, when Offset is small.

On z13, if eliminateFrameIndex() chooses LE (and not LEY), immediately
transform that LE to LDE32 to avoid partial register dependencies.

LEY should be generally preferred for big offsets over an expansion
into LAY + LDE32.

Reviewed by Ulrich Weigand.

llvm-svn: 266060