Split x86's ADJCALLSTACK instructions into 32-bit and 64-bit forms.

This allows the 64-bit forms to use+def RSP instead of ESP. This
doesn't fix any real bugs today, but it is more precise and it
makes the debug dumps on x86-64 look more consistent.

Also, add some comments describing the CALL instructions' physreg
operand uses and defs.

llvm-svn: 56925