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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56925 91177308-0d34-0410-b5e6-96231b3b80d8