Fix 7a6f294f.

The first correction enables correct execution DoMathMinMax when two
input registers are the same register.
The second correction adds NOP instructions after branch instructions
in tests macro_float_minmaxf(32|64).

TEST=cctest/test-macro-assembler-mips[64]/macro_float_minmax_f32
     cctest/test-macro-assembler-mips[64]/macro_float_minmax_f64
     mjsunit/regress/math-min
BUG=

Review-Url: https://codereview.chromium.org/2556793003
Cr-Commit-Position: refs/heads/master@{#41596}