LLVM JIT: Increase optimisation opportunities in update_count

Calculating the length of the range of instructions in words with
'sub nuw' (subtract, no unsigned wrap) and 'lshr exact' (logical shift
right, no 1s shifted out) presents more opportunities for optimisation
in the LLVM IR.
LLVM JIT: Simplify branches to the false case of B* opcodes

Instead of branching to a block that will branch to the block backing
the opcode at PC + 8, the branch is made to PC + 8 directly.

Shortcutting the branch can be done by LLVM, but having fewer basic
blocks to begin with can reduce the time required to compile functions.