[PGO] Early detection regarding whether pgo counter promotion is possible

Patch by Chris Chrulski

This fixes a problem with the current behavior when assertions are enabled.
A loop that exits to a catchswitch instruction is skipped for the counter
promotion, however this check was being done after the PGOCounterPromoter
tried to collect an insertion point for the exit block. A call to
getFirstInsertionPt() on a block that begins with a catchswitch instruction
triggers an assertion. This change performs a check whether the counter
promotion is possible prior to collecting the ExitBlocks and InsertPts.

Differential Revision: https://reviews.llvm.org/D73222