[AMDGPU] Only allow implicit WQM in pixel shaders

Implicit derivatives are only valid in pixel shaders,
hence only implicitly enable WQM for pixel shaders.
This avoids unintended WQM in other shader types (e.g. compute)
when image sampling instructions are used.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D114414