Try to remove Instruction::branch_{i32,i64}_{nez,eqz}

[Robbepop]

With recent refactoring and improvements to the register-machine wasmi engine we ended up having

• Instruction::branch_i32_eqz
• Instruction::branch_i32_nez
• Instruction::branch_i64_eqz
• Instruction::branch_i64_nez

Which are superseeded by the also existing:

• Instruction::branch_i32_eq_imm16
• Instruction::branch_i32_ne_imm16
• Instruction::branch_i64_eq_imm16
• Instruction::branch_i64_ne_imm16

The only downside is that the latter 4 instructions cannot deal with 32-bit BranchOffset and use a 16-bit BranchOffset instead, however, given that I have not yet encountered a single Wasm program that even comes close to approach this limit I think it is okay to ditch reliance on 32-bit BranchOffset.

Removing the 4 superseeded instructions would clean up the engine a bit.