Move PathV2.h to Path.h

Most clients have already been moved from Path V1 to V2. The ones using V1
now include PathV1.h explicitly.
[mips] Add an IR transformation pass that optimizes calls to sqrt.

The pass emits a call to sqrt that has attribute "read-none". This call will be
converted to an ISD::FSQRT node during DAG construction, which will turn into
a mips native sqrt instruction.