[ARM] Make sure to save/restore LR when we use tBfar.

This change does two things. One, it ensures compilation will abort
instead of miscompiling if ARMFrameLowering::determineCalleeSaves
chooses not to save LR in a case where it's necessary.  Two, it changes
the way we estimate the size of a function to be more conservative in
the presence of constant pool entries and jump tables.

EstimateFunctionSizeInBytes probably still isn't really conservative
enough, but I'm not sure how we can come up with a reliable estimate
before constant islands runs.

Differential Revision: https://reviews.llvm.org/D59439

llvm-svn: 356527