Encoding fixes needed for multi-precision instructions

[ben-marshall]

An overlap in opcodes introduced by #52 and #63 was missed by the script which checks for such things. sigh.

I'll re-arrange the encodings for the multi-precision and LUT instructions to remove the ambiguity.

• Re-build of binutils will be required.
• All formal checks will be re-run before this issue is closed.

[ben-marshall]

This fix will accompany a new release.