[DAGCombine][X86] Pull one-use `freeze` out of `extract_vector_elt` vector operand

This may allow us to further simplify the vector,
and freezing the extracted result is still fine:
```
----------------------------------------
define i8 @src(<2 x i8> %src, i64 %idx) {
%0:
  %i1 = freeze <2 x i8> %src
  %i2 = extractelement <2 x i8> %i1, i64 %idx
  ret i8 %i2
}
=>
define i8 @tgt(<2 x i8> %src, i64 %idx) {
%0:
  %i1 = extractelement <2 x i8> %src, i64 %idx
  %i2 = freeze i8 %i1
  ret i8 %i2
}
Transformation seems to be correct!
```

BUT, there must not be other uses of that freeze,
see `@freeze_extractelement_extra_use`.

Also, looks like we are missing some ISEL-level handling for freeze.