[tblgen] Add a timer covering the time spent reading the Instruction defs

[tblgen] Add a timer covering the time spent reading the Instruction defs

This patch adds a -time-regions option to tablegen that can enable timers
(currently only one) that assess the performance of tablegen itself. This
can be useful for identifying scaling problems with tablegen backends.

This particular timer has allowed me to ignore time that is not attributed
the GISel combiner pass. It's useful by itself but it is particularly
useful in combination with https://reviews.llvm.org/D52954 which causes
this period of time to be annotated within Xcode Instruments which in turn
allows profile samples and recorded allocations attributed to reading
instructions to be filtered out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353763 91177308-0d34-0410-b5e6-96231b3b80d8