- Adds vmin, vmax for FP and integer vectors, both signed and unsigned.
- Regularizes switching logic in disasm and simulator for special codes
4 and 6.
- Factors vrecpe, vrsqrte, vrecps, vrsqrts into helper fns.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2623993006
Cr-Commit-Position: refs/heads/master@{#42385}