[SystemZ] Use DSGFR over DSGR in more cases

Fixes some cases where we were using full 64-bit division for (sdiv i32, i32)
and (sdiv i64, i32).

The "32" in "SDIVREM32" just refers to the second operand.  The first operand
of all *DIVREM*s is a GR128.

llvm-svn: 185435