These map to single Intel instructions.
The i64 to float conversions are not tested yet. The encoding tables
can't yet differentiate instructions on a secondary type variable alone.
A fallthrough jump is actually represented as 0 bytes, so no encoding is
needed.
Also allow for unencoded instructions in the generated emit_inst
implementations. The verifier has stricter rules for when this is
allowed.
Change the result type for the bit-counting instructions from a fixed i8
to the iB type variable which is the type of the input. This matches the
convention in WebAssembly, and at least Intel's instructions will set a
full register's worth of count result, even if it is always < 64.
Duplicate the Intel 'ur' encoding recipe into 'umr' and 'urm' variants
corresponding to the RM and MR encoding variants. The difference is
which register is encoded as 'reg' and which is 'r/m' in the ModR/M
byte. A 'mov' register copy uses the MR variant, a unary popcnt uses the
RM variant.
