[VectorCombine] Fix crash when folding select of bitcast (#177183)

Fixes #177144. Nits appreciated.

The fold in question does the following transformation:
Before
```
%bc = bitcast <4 x i32> %src to <16 x i8>
%e0 = extractelement <16 x i8> %bc, i32 0
%s0 = select i1 %cond, i8 %e0, i8 0
%e1 = extractelement <16 x i8> %bc, i32 1
%s1 = select i1 %cond, i8 %e1, i8 0
...
```

After
```
%sel = select i1 %cond, <4 x i32> %src, <4 x i32> zeroinitializer
%bc = bitcast <4 x i32> %sel to <16 x i8>
%e0 = extractelement <16 x i8> %bc, i32 0
%e1 = extractelement <16 x i8> %bc, i32 1
...
```
If every select shares the condition and has 0 in the false branch, the
bitcast can be replaced with a select between the original vector and
`zeroinitializer`, followed by a bitcast. Then each `select(cond,
extelt(...), 0)` can be replaced with `extelt(...)`.
The crash happens when the condition is defined after the original
bitcast, because the bitcast is replaced with the select + bitcast, and
now the select references a condition not yet defined.