Set TYPE_BRANCH for x(PC) in the parser and the assembler has less work to do.
This also makes the operand test handle -4(PC) correctly.

Also add a special test case for AX:DX, which should be fixed in obj really.

Change-Id: If195e3a8cf3454a73508633e9b317d66030da826
Reviewed-on: https://go-review.googlesource.com/5071Reviewed-by: Russ Cox <rsc@golang.org>