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.

llvm-svn: 268955