[InstCombine] Remove dbg.values describing contents of dead allocas

[vedantk]

When InstCombine removes an alloca, it erases the dbg.{addr,declare}
instructions which refer to the alloca. It would be better to instead
remove all debug intrinsics which describe the contents of the dead
alloca, namely all dbg.value(, ..., DW_OP_deref)'s.

This effectively undoes work performed in an InstCombine run earlier in
the pipeline by LowerDbgDeclare, which inserts DW_OP_deref dbg.values
before CallInst users of an alloca. The motivating example looks like:

  define void @foo(i32 %0) {
    %a = alloca i32              ; This alloca is erased.
    store i32 %0, i32* %a
    dbg.value(i32 %0, "arg0")    ; This dbg.value survives.
    dbg.value(i32* %a, "arg0", DW_OP_deref)
    call void @trivially_inlinable_no_op(i32* %a)
    ret void
  }

If the DW_OP_deref dbg.value is not erased, it becomes dbg.value(undef)
after inlining, making "arg0" unavailable. But we already have dbg.value
descriptions of the alloca's value (from LowerDbgDeclare), so the
DW_OP_deref dbg.value cannot serve its purpose of describing an
initialization of the alloca by some callee. It invalidates other useful
dbg.values, causing large gaps in location coverage, so we should delete
it (even though doing so may cause stale dbg.values to appear, if
there's a dead store to %a in @trivially_inlinable_no_op).

OTOH, it wouldn't be correct to delete all dbg.value descriptions of an
alloca. Note that it's possible to describe a variable that takes on
different pointer values, e.g.:

  void use(int *);
  void t(int a, int b) {
    int *local = &a;     // dbg.value(i32* %a.addr, "local")
    local = &b;          // dbg.value(i32* undef, "local")
    use(&a);             //           (note: %b.addr is optimized out)
    local = &a;          // dbg.value(i32* %a.addr, "local")
  }

In this example, the alloca for "b" is erased, but we need to describe
the value of "local" as before the call to "use". This
prevents "local" from appearing to be equal to "&a" at the callsite.

rdar://66592859

Differential Revision: https://reviews.llvm.org/D85555

(cherry picked from commit 3419252)

[vedantk]

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