[SystemZ] Pass regalloc hints to help Load-and-Test transformations.

Since there is no "Load-and-Test-High" instruction, the 32 bit load of a
register to be compared with 0 can only be implemented with LT if the virtual
GRX32 register ends up in a low part (GR32 register).

This patch detects these cases and passes the GR32 registers (low parts) as
(soft) hints in getRegAllocationHints().

Review: Ulrich Weigand.
llvm-svn: 354935