Currently, we are using the following sequence for load/store
with large offset (offset > 16b):
    lui at, 0x1234
    ori at, at, 0x5678
    add at, s0, at
    lw a0, 0(at)

This sequence can be optimized in the following way:
    lui at, 0x1234
    add at, s0, at
    lw a0, 0x5678(at)

BUG=

Review-Url: https://codereview.chromium.org/2503493002
Cr-Commit-Position: refs/heads/master@{#40988}