x86: improve matching diagnostics when "accumulator" registers are involved

x86: improve matching diagnostics when "accumulator" registers are involved

In templates, the expectation of an "accumulator" register to be used is
expressed solely by operand size; there's no "class" specifier there.
Hence operand_size_match() is too eager in invoking
match_{operand,simd}_size(), resulting in "operand size mismatch" errors
when it's the type (of register), not the size that's wrong.
Interestingly adjustments there alone lead to no error at all then: To
"compensate", operand_type_match() needs to disambiguate register types
when register instances are specified in the template (matching the
actual operand), by checking a match (overlap) in operand sizes.