[FuncSpec] Skip SCCP on blocks of dead functions and poison their callsites (#154668)
Fixes #153295.
For test case below:
```llvm
define i32 @caller() {
entry:
%call1 = call i32 @callee(i32 1)
%call2 = call i32 @callee(i32 0)
%cond = icmp eq i32 %call2, 0
br i1 %cond, label %common.ret, label %if.then
common.ret: ; preds = %entry
ret i32 0
if.then: ; preds = %entry
%unreachable_call = call i32 @callee(i32 2)
ret i32 %unreachable_call
}
define internal i32 @callee(i32 %ac) {
entry:
br label %ai
ai: ; preds = %ai, %entry
%add = or i32 0, 0
%cond = icmp eq i32 %ac, 1
br i1 %cond, label %aj, label %ai
aj: ; preds = %ai
ret i32 0
}
```
Before specialization, the SCCP solver determines that
`unreachable_call` is unexecutable, as the value of `callee` can only be
zero.
After specializing the call sites `call1` and `call2`, FnSpecializer
announces `callee` is a dead function since all executable call sites
are specialized. However, the unexecutable call sites can become
executable again after solving specialized calls.
In this testcase, `call2` is considered `Overdefined` after
specialization, making `cond` also `Overdefined`. Thus,
`unreachable_call` becomes executable.
This patch skips SCCP on the blocks in dead functions, and poisons the
call sites of dead functions.
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