[AMDGPU] Mark PC_ADD_REL_OFFSET rematerializable

Currently machine LICM hoist PC_ADD_REL_OFFSET out of loops,
causes register pressure when function calls are deep in
loops. This is a main cause of sgpr spill for programs
containing large number of function calls in loops.

This patch marks PC_ADD_REL_OFFSET as rematerializable,
which eliminates sgpr spills due to function calls
in loops. It significantly improves performance of
HIP apps which have large number of function calls,
e.g., Blender.

Author: yxsamliu

llvm/lib/Target/AMDGPU/SIInstructions.td

@@ -1036,6 +1036,7 @@ let isConvergent = 1 in {
   defm SI_SPILL_WWM_AV32 : SI_SPILL_VGPR <AV_32, 1>;
 }
 
+let isReMaterializable = 1, isAsCheapAsAMove = 1 in
 def SI_PC_ADD_REL_OFFSET : SPseudoInstSI <
   (outs SReg_64:$dst),
   (ins si_ga:$ptr_lo, si_ga:$ptr_hi),