Fix bad copy-pasted assert in the api.ir test.
Fix bad copy-pasted assert in the api.ir test.

XDI, XSI, XSP, and XBP all have 16-bit sub-registers, it's just the
8-bit ones that were used for AH, etc.

git-svn-id: ed337ca7cdb08c0ba29d8fe58632bb7686892c2d@1330 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6

rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio,rnk/dynamorio