[AMDGPU] Fix AddedComplexity for s_buffer_load patterns. NFCI.

We set AddedComplexity = 100 for s_load patterns to prefer them over
global loads, but for s_buffer_load patterns there is no need to do
this and it was quietly overriding the AddedComplexity of each
individual GCNPat that is defined inside SMLoad_Pattern (but in practice
that did not appear to make any difference).

Differential Revision: https://reviews.llvm.org/D145396