move a bunch of register constraints from being handled by

move a bunch of register constraints from being handled by
getRegClassForInlineAsmConstraint to being handled by
getRegForInlineAsmConstraint.  This allows us to let the llvm register allocator
allocate, which gives us better code.  For example, X86/2007-01-29-InlineAsm-ir.ll
used to compile to:

_run_init_process:
        subl $4, %esp
        movl %ebx, (%esp)
        xorl %ebx, %ebx
        movl $11, %eax
        movl %ebx, %ecx
        movl %ebx, %edx
        # InlineAsm Start
        push %ebx ; movl %ebx,%ebx ; int $0x80 ; pop %ebx
        # InlineAsm End

Now we get:
_run_init_process:
        xorl %ecx, %ecx
        movl $11, %eax
        movl %ecx, %edx
        # InlineAsm Start
        push %ebx ; movl %ecx,%ebx ; int $0x80 ; pop %ebx
        # InlineAsm End

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35804 91177308-0d34-0410-b5e6-96231b3b80d8