[X86] Add missing test cases for some int/fp->fp conversion intrinsics with...

[X86] Add missing test cases for some int/fp->fp conversion intrinsics with rounding mode. Use non-default rounding mode on some tests.

For some reason we were missing tests for several unmasked conversion intrinsics, but had their mask form.

Also use a non-default rounding mode on some tests to provide better coverage for a future patch.

llvm-svn: 351708