x86: accept whitespace around prefix separator

... and prediction suffix comma. Other than documented /**/ comments
currently aren't really converted to a single space, at least not for
x86 in its most common configurations. That'll be fixed subsequently, at
which point blanks may appear where so far none were expected.
Furthermore not permitting blanks around these separators wasn't quite
logical anyway - such constructs are composite ones, and hence
components ought to have been permitted to be separated by whitespace
from the very beginning. Furthermore note how, due to the scrubber being
overly aggressive in removing whitespace, some similar construct with a
prefix were already accepted.

Note how certain other checks in parse_insn() can be simplified as a
result.

While there for the prediction suffix also make checks case-insensitive
and check for a proper trailing separator.