x86: improve matching diagnostics

x86: improve matching diagnostics

Many times in the past I was puzzled by seeing "operand size mismatch"
when really "operand type mismatch" would be far more appropriate. As it
turns out, there were at least two flaws: In the single operand case we
didn't propagate i.error to match_template()'s local specific_error when
noticing a type mismatch. And then operand_size_match() was too eager in
invoking match_mem_size(): Especially the Unspecified attribute can get
in the way there when the expected operand isn't a memory one (and hence
Unspecified would not be set in the operand template, whereas it's
uniformly set for memory operands in AT&T syntax).

(In the x86-64-lkgs-inval testcase the particular error for the two
bogus Intel syntax forms doesn't really matter; all we ought to care
about there isthat there is _some_ error.)