Refactor inline threshold update code.

Functional change: Previously, if a callee is cold, we used ColdThreshold if it minimizes the existing threshold. This was irrespective of whether we were optimizing for minsize (-Oz) or not. But -Oz uses very low threshold to begin with and the inlining with -Oz is expected to be tuned for lowering code size, so there is no good reason to set an even lower threshold for cold callees. We now lower the threshold for cold callees only when -Oz is not used. For default values of -inlinethreshold and -inlinecold-threshold, this change has no effect and this simplifies the code.

NFC changes: Group all threshold updates that are guarded by !Caller->optForMinSize() and within that group threshold updates that require profile summary info.

Differential revision: https://reviews.llvm.org/D28369

llvm-svn: 291487