[AMDGPU] Mark sin/cos load folding as modifying the function.

When the load value is folded into the sin/cos operation, the
AMDGPU library calls simplifier could still mark the function
as unmodified. Instead ensure if there is an early return,
return whether the load was folded into the sin/cos call.

Authored by MJDSys

Differential Revision: https://reviews.llvm.org/D91401