[LSR][ARM] Add new TTI hook to mark some LSR chains as profitable

This patch adds a new TTI hook to allow targets to tell LSR that
a chain including some instruction is already profitable and
should not be optimized. This patch also adds an implementation
of this TTI hook for ARM so LSR doesn't optimize chains that include
the VCTP intrinsic.

Differential Revision: https://reviews.llvm.org/D79418