[GlobalISel] Make the InstructionSelector instance non-const, allowing state to be maintained.

Currently we can't keep any state in the selector object that we get from
subtarget. As a result we have to plumb through all our variables through
multiple functions. This change makes it non-const and adds a virtual init()
method to allow further state to be captured for each target.

AArch64 makes use of this in this patch to cache a call to hasFnAttribute()
which is expensive to call, and is used on each selection of G_BRCOND.

Differential Revision: https://reviews.llvm.org/D65984

llvm-svn: 368652