The original code legalized *all* i64 constants into a register move, creating unnecessary instructions and slightly higher register pressure in most cases.

Generally, immediates can be used in 64-bit instructions as long as the immediate can be represented as a sign-extended 32-bit value.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2063053002 .