Cleanup register pressure calculation in MachineLICM.

There were four almost identical implementations of calculating/updating
the register pressure for a certain MachineInstr. Cleanup to have a
single implementation (well, controlled with two bool flags until this
is cleaned up more).

No functional changes intended.

Tested by verify that there are no binary changes in the entire llvm
test-suite. A new test was added separately in r234309 as it revealed a
pre-existing error in the register pressure calculation.

llvm-svn: 234325