After Cvt_d_uw macro, upper 32 bits of the output remain
unitnitialized which caused flaky failures on some tests on
MIPS32R6

TEST=cctest/test-assembler-mips/MIPS13,mjsunit/asm/int32-umod
BUG=

Review URL: https://codereview.chromium.org/1537973002

Cr-Commit-Position: refs/heads/master@{#32983}