[X86][AsmParser] In Intel syntax make sure we support ESP/RSP being the second...

[X86][AsmParser] In Intel syntax make sure we support ESP/RSP being the second register in memory expressions like [EAX+ESP].

By default, the second register gets assigned to the index register slot. But ESP can't be an index register so we need to swap it with the other register.

There's still a slight bug that we allow [EAX+ESP*1]. The existence of the multiply even though its with 1 should force ESP to the index register and trigger an error, but it doesn't currently.

llvm-svn: 335394