[AMDGPU] Fix ieee mode default value

Previously, the default value for ieee mode was
- on for compute kernels and compute shaders,
- off for all shaders except compute shaders.

This commit changes the default to be
- on for compute kernels,
- off for shaders.

This aligns the default value with the settings that are actually in
use.  To my knowledge, all users of shader calling conventions (mesa and
llpc) disable the ieee mode by default.

Differential Revision: https://reviews.llvm.org/D89388