[CGP] Drop poison-generating flags after hoisting (#90382)
See the following case:
```
define i8 @src1(i8 %x) {
entry:
%cmp = icmp eq i8 %x, -1
br i1 %cmp, label %exit, label %if.then
if.then:
%inc = add nuw nsw i8 %x, 1
br label %exit
exit:
%retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
ret i8 %retval
}
define i8 @tgt1(i8 %x) {
entry:
%inc = add nuw nsw i8 %x, 1
%0 = icmp eq i8 %inc, 0
br i1 %0, label %exit, label %if.then
if.then: ; preds = %entry
br label %exit
exit: ; preds = %if.then, %entry
%retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
ret i8 %retval
}
```
`optimizeBranch` converts `icmp eq X, -1` into cmp to zero on RISC-V and
hoists the add into the entry block. Poison-generating flags should be
dropped as they don't still hold.
Proof: https://alive2.llvm.org/ce/z/sP7mvK
Fixes https://github.com/llvm/llvm-project/issues/90380
ab12bba0aa