[CSSPGO] Unblock optimizations with pseudo probe instrumentation part 3.

This patch continues unblocking optimizations that are blocked by pseudo probe instrumentation.

Not exactly like DbgIntrinsics, PseudoProbe intrinsic has other attributes (such as mayread, maywrite, mayhaveSideEffect) that can block optimizations. The issues fixed are:
- Flipped default param of getFirstNonPHIOrDbg API to skip pseudo probes
- Unblocked CSE by avoiding pseudo probe from clobbering memory SSA
- Unblocked induction variable simpliciation
- Allow empty loop deletion by treating probe intrinsic isDroppable
- Some refactoring.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D110847