[llvm] [GlobalMerge] Aggressively merge constants to reduce TOC entries (PR #111756)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

```
static const int a[4000];
static const int b[4000];
static const int c[3000];
static const int d[3000];

int foo(int *, int*);

int int1() {
 return foo(a, c);
}

int int2() {
 return foo(b, d);
}

int int3() {
 return foo(a, c);
}

int int4() {
 return foo(b, d);
}
```

If you put "a" and "c" together, and "b" and "d" together, each function has one TOC load.  If you merge a+b and c+d instead, each function has two TOC loads.

https://github.com/llvm/llvm-project/pull/111756