The higher word (bits 32 - 63) of FPU register is set on zero before
storing result.

TEST=cctest/test-macro-assembler-mips64/Cvt_s_uw_Trunc_uw_s
BUG=

Review URL: https://codereview.chromium.org/1812193003

Cr-Commit-Position: refs/heads/master@{#34889}