Singlepass: refactor condition jumps to single fn (jmp_on_condition)

[marxin]

For the purpose of the upcoming RISC-V support (#5711), I would like to merge the location_cmp/emit_relaxed_cmp and jmp_on_$condition into a single function. Mainly motivated by the fact the RISC-V architecture does not profile the FLAGS register used for the jump instruction. On the other hand, it provides direct compare and jump instructions that, with register operations, directly map to the newly introduced jmp_on_condition. Right now, the RISC-V PR contains unnecessary changes where a new temporary register is first used for comparison, and then jmp_on_true/false function do the actual jump.

Note the currently used approach might be seen tricky as the codegen is responsible that no other FLAGS-modifying instructions sneaks in between the CMP and the consequent JUMP instruction.

[syrusakbary]

This looks good, but I'd like to merge after the 6.1.0 final release