Reapply [X86] Add a new LOW32_ADDR_ACCESS_RBP register class.
This reapplies commit r268796, with a fix for the setting of the inline asm
constraints. I.e., "mark" LOW32_ADDR_ACCESS_RBP as a GR variant, so that the
regular processing of the GR operands (setting of the subregisters) happens.
Original commit log:
[X86] Add a new LOW32_ADDR_ACCESS_RBP register class.
ABIs like NaCl uses 32-bit addresses but have 64-bit frame.
The new register class reflects those constraints when choosing a
register class for a address access.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268955
91177308-0d34-0410-b5e6-
96231b3b80d8