The current code for testing the VEX.L flag, indicating whether
128-bit or 256-bit registers are being accessed, was erroneous
and always returned true (i.e. indicated 128-bit registers).

This patch fixes this behaviour and checks the flag correctly.

Ref: https://github.com/nodejs/node/issues/6151

BUG=

Review URL: https://codereview.chromium.org/1875323002

Cr-Commit-Position: refs/heads/master@{#35506}