This patch replaces all dominated uses of condition with true/false to
improve context-sensitive optimizations. It eliminates a bunch of
branches in llvm-opt-benchmark.
As a side effect, it may introduce new phi nodes in some corner cases.
See the following case:
```
define i1 @test(i1 %cmp, i1 %cond) {
entry:
br i1 %cond, label %bb1, label %bb2
bb1:
br i1 %cmp, label %if.then, label %if.else
if.then:
br %bb2
if.else:
br %bb2
bb2:
%res = phi i1 [%cmp, %entry], [%cmp, %if.then], [%cmp, %if.else]
ret i1 %res
}
```
It will be simplified into:
```
define i1 @test(i1 %cmp, i1 %cond) {
entry:
br i1 %cond, label %bb1, label %bb2
bb1:
br i1 %cmp, label %if.then, label %if.else
if.then:
br %bb2
if.else:
br %bb2
bb2:
%res = phi i1 [%cmp, %entry], [true, %if.then], [false, %if.else]
ret i1 %res
}
```
I am planning to fix this in late pipeline/CGP since this problem exists
before the patch.
380fa875ab