MIPS: Fix ambiguous double constant loading after adding checked load/store operators.

The V8_INFINITY macro expands to constant expression of type float.

TEST=mjsunit/constant-folding-2
BUG=
R=balazs.kilvady@imgtec.com

Review URL: https://codereview.chromium.org/779433003

Cr-Commit-Position: refs/heads/master@{#25635}