RegisterPressure: Remove 0 entries from PressureChange

RegisterPressure: Remove 0 entries from PressureChange

This should not change behaviour because as far as I can see all code
reading the pressure changes has no effect if the PressureInc is 0.
Removing these entries however does avoid unnecessary computation, and
results in a more stable debug output. I want the stable debug output to
check that some upcoming changes are indeed NFC and identical even at
the debug output level.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250595 91177308-0d34-0410-b5e6-96231b3b80d8