Failure from pext on x86-64
`test-primops` generates the following program which fails with a floating-point exception on x86-64/Linux:
```
zext[W16→W64](0::W16 /u ((((zext[W8→W16](0x12::W8 + (load[W8](0x17dd4::W64)))) & 0x59f8::W16) << 0xa::W64) /s 0xffff::W16))
```
I'm not yet sure whether this is a `test-primops` issue or a GHC bug.
## `test.cmm`
```
#include "Cmm.h"
test ( bits64 buffer ) {
bits64 ret;
(ret) = prim %pext64(
%zx64(%divu(
(0 :: bits16),
%quot(
%shl(
(%zx16(((18 :: bits8) + bits8[buffer + (97748 :: bits64)])) & (23032 :: bits16)),
(10 :: bits64)
),
(-1 :: bits16))
)),
(0 :: bits64)
);
return (ret);
}
```
issue