[RISCV] Add machine function pass to merge base + offset

Summary:
   In r333455 we added a peephole to fix the corner cases that result
   from separating base + offset lowering of global address.The
   peephole didn't handle some of the cases because it only has a basic
   block view instead of a function level view.

   This patch replaces that logic with a machine function pass. In
   addition to handling the original cases it handles uses of the global
   address across blocks in function and folding an offset from LW\SW
   instruction. This pass won't run for OptNone compilation, so there
   will be a negative impact overall vs the old approach at O0.

Reviewers: asb, apazos, mgrang

Reviewed By: asb

Subscribers: MartinMosbeck, brucehoult, the_o, rogfer01, mgorny, rbar, johnrusso, simoncook, niosHD, kito-cheng, shiva0217, zzheng, llvm-commits, edward-jones

Differential Revision: https://reviews.llvm.org/D47857

llvm-svn: 335786