Fix fixed-point vcvt_f64_s32 immediate value encoding

The (32 - fraction_bits) value should be encoded so that the least
significant bit is set to bit 5 and the four next bits to bits 0-3. Fix
the previously incorrect encoding. This bug did not cause behavioral
issues before, since in existing uses of the function the order of the
bits in the immediate value does not matter, as they are all 1.

BUG=3256
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/223623003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20508 ce2b1a6d-e550-0410-aec6-3dcde31c8c00