adjust decode logic for better pipelining

at least on my zen2.

when reading prefixes, optimize for the likely case of reading an
instruction rather than an invalid run of prefixes. checking if we've
exceeded the x86 length bound immediately after reading the byte is only
a benefit if we'd otherwise read an impossibly-long instruction; in this
case we can exit exactly at prefix byte 15 rather than potentially later
at byte 16 (assuming a one-byte instruction like `c3`), or byte ~24 (a
more complex store with immediate and displacement).

these casese are extremely unlikely in practice. more likely is that
reading a prefix byte is one of the first two or three bytes in an
instruction, and we will never benefit from checking the x86 length
bound at this point. instead, only check length bounds after decoding
the entire instruction. this penalizes the slowest path through the
decoder but speeds up the likely path about 5% on my zen2 processor.

additionally, begin reading instruction bytes as soon as we enter the
decoder, and before initial clearing of instruction data. again, this is
for zen2 pipeline reasons. reading the first byte and corresponding
  `OPCODES` entry improves the odds that this data is available by the
time we check for `Interpretation::Prefix` in the opcode scanning
loop. then, if we did *not* load an instruction, we immediately know
another byte must be read; begin reading this byte before applying `rex`
prefixes, and as soon as a prefix is known to not be one of the
escape-code prefix byte (c5, c4, 62, 0f). this clocked in at another ~5%
in total.

i've found that `read_volatile` is necessary to force rust to begin the
loadwhere it's written, rather than reordering it over other data. i'm
not committed to this being a guaranteed truth.

also, don't bother checking for `Invalid`. again, `Opcode::Invalid` is a
relatively unlikely path through the decoder and `Nothing` is already
optiimized for `None` cases. this appears to be another small improvement
in throughput but i wouldn't want to give it a number - it was
relatively small and may not be attributable to this effect.

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