MIPS: Changed "marked" nops to use sll(zero_reg, at, type).

MIPS: Changed "marked" nops to use sll(zero_reg, at, type).

We use marking bits in nops (in the 'sa' field) for debug markers, and for some IC stuff. A normal NOP in mips is sll(zero_reg, zero_reg, 0), where the 0 is a 5 bit immediate field in 'sa'.

See enum NopMarkerTypes at around line 654 of assembler-mips.h

The problem is that these markers use encodings that are reserved for the 'ssnop' and 'ehb' instructions. These are instructions used for hazard barriers.

It does not break anything, but it will slow things down a little bit as some pipeline stages are cleared, etc.

This commit changes the "marked" NOPs to sll(zero_reg, at, type) instructions, which is also a NOP operation on MIPS.

BUG=
TEST=

Committed: https://code.google.com/p/v8/source/detail?r=12657