[AMDGPU] Boost unroll threshold for loops reading local memory

This is less important than increase threshold for private memory,
but still brings performance improvements in a wide range of tests.
Unrolling more for local memory serves three purposes: it allows
to combine ds operations if offset becomes static, saves registers
used for offsets in case of static offsets, and allows better lds
latency hiding.

Differential Revision: https://reviews.llvm.org/D31412

llvm-svn: 298948