Quick compiler: Fix MIPS build

In debug builds, the Quick compiler frequently runs a sanity checker
over the register pool.  Among other things, it attempts to verify
consistent representation of register pairs.  However, a register's
"wide" flag is meaningful only when the register pair is associated
with a Dalvik wide value (sreg != INVALID_SREG) rather than a temp
wide value.

The MIPS build was tripping over this bad assertion.  Fixed here.

Note related cl/105461

Change-Id: Id726ff1ea0f5cbcc8dba6fa3aacb3fd4fc043a63