[PowerPC] Fix erroneous condition for converting uint-to-fp vector conversion

A condition for exiting the legalization of v4i32 conversion to v2f64 through
extract/convert/build erroneously checks for the extract having type i32.
This is not adequate as smaller extracts are actually legalized to i32 as well.
Furthermore, an early exit is missing which means that we only check that
both extracts are from the same vector if that check fails.
As a result, both cases in the included test case fail - the first gets a
select error and the second generates incorrect code.

The culprit commit is r274535.

llvm-svn: 360043