When doing select-to-zext/sext transformations, we should not handle
TrueVal and FalseVal of i1 type otherwise it would result in zext/sext i1 to i1.
Note that the current trunk code does not break since the checking currently does exist
but is implicit. When we have TrueVal and FalseVal of i1 type, this case will be captured
in the previous code block:
Therefore no unit test is provided in this patch. For maintenance purposes however,
this patch makes the checking explicit instead of implicit.