[Matrix] Implement += and -= for MatrixType.

Make sure CompLHSTy is set correctly for += and -= and matrix type
operands.

Bugzilla ticket is here https://bugs.llvm.org/show_bug.cgi?id=46164

Patch by Saurabh Jha <[email protected]>

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D98075

Author: SaurabhJha

clang/lib/Sema/SemaExpr.cpp

@@ -10551,7 +10551,11 @@ QualType Sema::CheckAdditionOperands(ExprResult &LHS, ExprResult &RHS,
 
   if (LHS.get()->getType()->isConstantMatrixType() ||
       RHS.get()->getType()->isConstantMatrixType()) {
-    return CheckMatrixElementwiseOperands(LHS, RHS, Loc, CompLHSTy);
+    QualType compType =
+        CheckMatrixElementwiseOperands(LHS, RHS, Loc, CompLHSTy);
+    if (CompLHSTy)
+      *CompLHSTy = compType;
+    return compType;
   }
 
   QualType compType = UsualArithmeticConversions(
@@ -10651,7 +10655,11 @@ QualType Sema::CheckSubtractionOperands(ExprResult &LHS, ExprResult &RHS,
 
   if (LHS.get()->getType()->isConstantMatrixType() ||
       RHS.get()->getType()->isConstantMatrixType()) {
-    return CheckMatrixElementwiseOperands(LHS, RHS, Loc, CompLHSTy);
+    QualType compType =
+        CheckMatrixElementwiseOperands(LHS, RHS, Loc, CompLHSTy);
+    if (CompLHSTy)
+      *CompLHSTy = compType;
+    return compType;
   }
 
   QualType compType = UsualArithmeticConversions(