[RegisterCoalescer] Fix SUBREG_TO_REG handling in the RegisterCoalescer. (#96839)

The issue with the handling of the SUBREG_TO_REG is that we don't join
the subranges correctly when we join live ranges across the
SUBREG_TO_REG. For example when joining across this:
```
32B	  %2:gr64_nosp = SUBREG_TO_REG 0, %0:gr32, %subreg.sub_32bit
```
we want to join these live ranges:
```
%0 [16r,32r:0) 0@16r  weight:0.000000e+00
%2 [32r,112r:0) 0@32r  weight:0.000000e+00
```
Before the fix the range for the resulting merged `%2` is:
```
%2 [16r,112r:0) 0@16r  weight:0.000000e+00
```
After the fix it is now this: 
```
%2 [16r,112r:0) 0@16r  L000000000000000F [16r,112r:0) 0@16r  weight:0.000000e+00
```

Two tests are added to this fix. The X86 test fails without the patch.
The PowerPC test passes with and without the patch but is added as a way
track future possible failures when register classes are changed in a
future patch.

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index 1c35a88..043ea20 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -3673,6 +3673,13 @@ bool RegisterCoalescer::joinVirtRegs(CoalescerPair &CP) {
 
     LHSVals.pruneSubRegValues(LHS, ShrinkMask);
     RHSVals.pruneSubRegValues(LHS, ShrinkMask);
+  } else if (TrackSubRegLiveness && !CP.getDstIdx() && CP.getSrcIdx()) {
+    LHS.createSubRangeFrom(LIS->getVNInfoAllocator(),
+                           CP.getNewRC()->getLaneMask(), LHS);
+    mergeSubRangeInto(LHS, RHS, TRI->getSubRegIndexLaneMask(CP.getSrcIdx()), CP,
+                      CP.getDstIdx());
+    LHSVals.pruneMainSegments(LHS, ShrinkMainRange);
+    LHSVals.pruneSubRegValues(LHS, ShrinkMask);
   }
 
   // The merging algorithm in LiveInterval::join() can't handle conflicting