[AMDGPU] Add use check in v_fma combine.

In D132837, an existing v_fma combine was extended to regard nested
fma instructions. Originally, the inner FMA was checked for being used
only once. In its current state, this check is missing, which causes
some regressions.

In this patch, this check was added.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D134856