RegisterPressure: Fix default lanemask for missing regunit intervals

In case of missing live intervals for a physical registers
getLanesWithProperty() would report 0 which was not a safe default in
all situations. Add a parameter to pass in a safe default.
No testcase because in-tree targets do not skip computing register unit
live intervals.

Also cleanup the getXXX() functions to not perform the
RequireLiveIntervals checks anymore so we do not even need to return
safe defaults.

llvm-svn: 267977