Removed REX.W requirement for several instructions

Summary: Several SSE related instructions didn't require REX.W, but we were
generating them anyways. Now we don't generate a REX prefix if if these
instructions use xmm0-xmm7. If they do use xmm8-xmm15, we emit the REX prefix
byte with REX.R set (which we already did correctly).

Reviewed By: @edwinsmith

Differential Revision: D1548397
Fix potential security leak in HashContext

Summary: CVE-2014-6229

This is not a NUL-terminated string, it's a fixed-size block of data.
The risks were key truncation (if there happens to be a NUL byte in the
key) or over-reading (which would be information leakage).

Reviewed By: @ptarjan

Differential Revision: D1533546