[RegAlloc] Allow rematerialization with virtual reg uses

Stacked on #159180.

Unless overridden by the target, we currently only allow rematerlization of instructions with immediate or constant physical register operands, i.e. no virtual registers.

The comment states that this is because we might increase a live range of the virtual register, but we don't actually do this. LiveRangeEdit::allUsesAvailableAt makes sure that we only rematerialize instructions whose virtual registers are already live at the use sites.

This patch relaxes this constraint which reduces a significant amount of reloads across various targets.

This is another attempt at https://reviews.llvm.org/D106408, but #159180 aims to have addressed the issue with the weights that may have caused the previous regressions.

Author: lukel97

llvm/lib/CodeGen/TargetInstrInfo.cpp

@@ -1657,12 +1657,6 @@ bool TargetInstrInfo::isReMaterializableImpl(
     // same virtual register, though.
     if (MO.isDef() && Reg != DefReg)
       return false;
-
-    // Don't allow any virtual-register uses. Rematting an instruction with
-    // virtual register uses would length the live ranges of the uses, which
-    // is not necessarily a good idea, certainly not "trivial".
-    if (MO.isUse())
-      return false;
   }
 
   // Everything checked out.