[MachineLICM] Use newer model of register pressure sets.

[MachineLICM] Use newer model of register pressure sets.

TargetRegisterInfo::getRegPressureLimit has a note that it is an old
model that relies on manually entered classes. Using the newer model of
register pressure sets seems more appropriate. We might eventually even
switch to lib/CodeGen/RegisterPressure.cpp, but we should probably do
incremental changes here.

Using the newer model also makes it easier to take regmasks into account
which is necessary to fix llvm.org/PR23143. I am currently also
preparing a patch for that, but would like to do this switch
independently.

Review: http://reviews.llvm.org/D8986
llvm-svn: 234880