[xray] Add xray-ignore-loops option

XRay allows tuning by minimum function size, but also always instruments
functions with loops in them.  If the minimum function size is set to a
large value the loop instrumention ends up causing most functions to be
instrumented anyway.  This adds a new flag, xray-ignore-loops, to disable
the loop detection logic.

Differential Revision: https://reviews.llvm.org/D72659