llc: x86 atomic load i256 crash

[Liblor]

Bugzilla Link	50032
Version	trunk
OS	Linux
CC	@topperc,@RKSimon,@phoebewang,@rotateright

Extended Description

Even though an atomic load of i256 isn't possible on x86, it shouldn't crash ideally.

https://godbolt.org/z/6bccv3efq

%V = load atomic i256, i256* %a unordered, align 32

[JOE1994]

With the attached godbolt link, llc trunk no longer crashes while compiling the given input IR.
Please re-open if needed.