[RegisterPressure] NFC: Clean up RP handling for instructions with overlapping Def/Use (#109875)

The current RP handling for uses of an MI that overlap with defs is
confusing and unnecessary. Moreover, the lane masks do not accurately
model the liveness behavior of the subregs. This cleans things up a bit
and more accurately models subreg lane liveness by sinking the use
handling into subsent Uses loop.

The effect of this PR is to replace

A. `increaseRegPressure(Reg, LiveAfter, ~LiveAfter & LiveBefore)`

with 

B. `increaseRegPressure(Reg, LiveAfter, LiveBefore)`

Note that A (Defs loop) and B (Uses loop) have different definitions of
LiveBefore

A. `LiveBefore = (LiveAfter & ~DefLanes) | UseLanes`

and 

B. `LiveBefore =  LiveAfter | UseLanes`

Also note, `increaseRegPressure` will exit if `PrevMask` (`LiveAfter`
for both A/B) has any active lanes, thus these calls will only have an
effect if `LiveAfter` is 0.


A. NewMask = ~LiveAfter & ((LiveAfter & ~DefLanes) | UseLanes) => (1 &
UseLanes) => UseLanes = (0 | UseLanes) => (LiveAfter | UseLanes) =
NewMask B.

Author: jrbyrnes

llvm/lib/CodeGen/RegisterPressure.cpp

@@ -1060,18 +1060,12 @@ void RegPressureTracker::bumpUpwardPressure(const MachineInstr *MI) {
     LaneBitmask LiveBefore = (LiveAfter & ~DefLanes) | UseLanes;
 
     // There may be parts of the register that were dead before the
-    // instruction, but became live afterwards. Similarly, some parts
-    // may have been killed in this instruction.
+    // instruction, but became live afterwards.
     decreaseRegPressure(Reg, LiveAfter, LiveAfter & LiveBefore);
-    increaseRegPressure(Reg, LiveAfter, ~LiveAfter & LiveBefore);
   }
-  // Generate liveness for uses.
+  // Generate liveness for uses. Also handle any uses which overlap with defs.
   for (const RegisterMaskPair &P : RegOpers.Uses) {
     Register Reg = P.RegUnit;
-    // If this register was also in a def operand, we've handled it
-    // with defs.
-    if (getRegLanes(RegOpers.Defs, Reg).any())
-      continue;
     LaneBitmask LiveAfter = LiveRegs.contains(Reg);
     LaneBitmask LiveBefore = LiveAfter | P.LaneMask;
     increaseRegPressure(Reg, LiveAfter, LiveBefore);