[X86] Cleanup the lrint/llrint/lround/llround tests a bit.

We don't need tests for truncating the result. There's nothing
special about those truncates.

We can test llrint/llround for 64-bit and 32-bit targets in the same file.
Same with lrint/lround with i32 result result. lrint/lround with
64-bit result should only occur on a 64-bit target.

Add some missing tests for f80 conversions.