[InstCombine] adjust tests for fptoui + trunc; NFC

Tests were duplicated for undef-ness, but that's not necessary.
There were no tests for bfloat/double.

I deleted the fptosi tests for now because those will just
be a bitwidth adjustment to the fptoui tests once we settle
on the right set of tests.

I think it should just be 2 tests on either side of the
bitwidth boundary to allow a fold with a couple of extra
tests thrown in to show other type constraints are not
a factor.

Goes with:
D142093