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
ce54b22657