Interprocedural Register Allocation (IPRA) Analysis

Add an option to enable the analysis of MachineFunction register
usage to extract the list of clobbered registers.

When enabled, the CodeGen order is changed to be bottom up on the Call
Graph.

The analysis is split in two parts, RegUsageInfoCollector is the
MachineFunction Pass that runs post-RA and collect the list of
clobbered registers to produce a register mask.

An immutable pass, RegisterUsageInfo, stores the RegMask produced by
RegUsageInfoCollector, and keep them available. A future tranformation
pass will use this information to update every call-sites after
instruction selection.

Patch by Vivek Pandya <vivekvpandya@gmail.com>

Differential Revision: http://reviews.llvm.org/D20769

llvm-svn: 272403