Overhaul Cell SPU's addressing mode internals so that there are now

only two addressing mode nodes, SPUaform and SPUindirect (vice the
three previous ones, SPUaform, SPUdform and SPUxform). This improves
code somewhat because we now avoid using reg+reg addressing when
it can be avoided. It also simplifies the address selection logic,
which was the main point for doing this.

Also, for various global variables that would be loaded using SPU's
A-form addressing, prefer D-form offs[reg] addressing, keeping the
base in a register if the variable is used more than once.

llvm-svn: 46483