[Clang][CoverageMapping] Fix switch counter codegen compile time explosion

C++ generated code with huge amount of switch cases chokes badly while emitting
coverage mapping, in our specific testcase (~72k cases), it won't stop after hours.
After this change, the frontend job now finishes in 4.5s and shrinks down `@__covrec_`
by 288k when compared to disabling simplification altogether.

There's probably no good way to create a testcase for this, but it's easy to
reproduce, just add thousands of cases in the below switch, and build with
`-fprofile-instr-generate -fcoverage-mapping`.

```
enum type : int {
 FEATURE_INVALID = 0,
 FEATURE_A = 1,
 ...
};

const char *to_string(type e) {
  switch (e) {
  case type::FEATURE_INVALID: return "FEATURE_INVALID";
  case type::FEATURE_A: return "FEATURE_A";}
  ...
  }

```

Differential Revision: https://reviews.llvm.org/D126345

1 files changed, 7 insertions, 5 deletions

diff --git a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
index 94c2bee..f9e58fd 100644
--- a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
@@ -123,13 +123,15 @@ Counter CounterExpressionBuilder::simplify(Counter ExpressionTree) {
   return C;
 }
 
-Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS) {
-  return simplify(get(CounterExpression(CounterExpression::Add, LHS, RHS)));
+Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS, bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Add, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
-Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS) {
-  return simplify(
-      get(CounterExpression(CounterExpression::Subtract, LHS, RHS)));
+Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS,
+                                           bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Subtract, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
 void CounterMappingContext::dump(const Counter &C, raw_ostream &OS) const {