[NFC] Pass the instruction delta to emitInstrCountChangedRemark

Instead of counting the size of the entire module every time we run a pass,
pass along a delta instead and use that to emit the remark.

This means we only have to use (on average) smaller IR units to calculate
instruction counts. E.g, in a BB pass, we only need to look at the delta of
the BB instead of the delta of the entire module.

6/6

(This improved compile time for size remarks on sqlite3 + O2 significantly)

llvm-svn: 341250