Port 1abeb5a3

Original Commit Message:

     - Shift opcode numbers for asmjs-compat opcodes
     - Add --experimental-wasm-se flag to gate sign extension opccodes
     - Fix codegen for ia32 movsx instructions

R=gdeepti@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I3af97112b40d159f9ffc4f465768fc7832485f20
Reviewed-on: https://chromium-review.googlesource.com/893703Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#50969}