[CodeGen] Adjust global-split remat heuristic to match LICM (llvm#160709)

This heuristic was originally added in 40c4aa with the stated purpose of
avoiding global split on live long ranges created by MachineLICM
hoisting trivially rematerializable instructions. In the meantime,
various backends have introduced non-trivial rematerialization cases,
MachineLICM gained an explicitly triviality check, and we've reworked
our APIs to match naming wise. Let's move this heuristic back to truely
trivial remat only.

This is a functional change, though somewhat hard to hit. This change
will cause non-trivially rematerializable instructions to be globally
split more often. This is likely a good thing since non-trivial remat
may not be legal at all possible points in the live interval, but may
cost slightly more compile time.

I don't have a motivating example; I found it when reviewing the callers
of isRemMaterializable(MI).

Author: preames

llvm/lib/CodeGen/TargetRegisterInfo.cpp

@@ -67,7 +67,8 @@ bool TargetRegisterInfo::shouldRegionSplitForVirtReg(
   const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
   const MachineRegisterInfo &MRI = MF.getRegInfo();
   MachineInstr *MI = MRI.getUniqueVRegDef(VirtReg.reg());
-  if (MI && TII->isReMaterializable(*MI) && VirtReg.size() > HugeSizeForSplit)
+  if (MI && TII->isTriviallyReMaterializable(*MI) &&
+      VirtReg.size() > HugeSizeForSplit)
     return false;
   return true;
 }