JIT: implement empty($cv) fast path in function & tracing JIT

[ndossche]

We can simply defer to zend_jit_bool_jmpznz() which already generates optimized paths for many types.

Example PHP:

function f(int $i) {
    if (empty($i)) return 1;
    return 2;
}

Before (function JIT):

.L1:
	movl $0x414d88a0, %r15d          ; opline = ISSET_ISEMPTY_CV
	movabsq $0x55a80140fe00, %rax
	callq *%rax                      ; Call VM handler
	cmpl $0x414d88e0, %r15d          ; check if the handler branched
	jne .L2
	movq 0x10(%r14), %rax            ; return 1
	...
.L2:                                     ; return 2
	...

After (function JIT):

.L1:
	cmpq $0, 0x50(%r14)              ; empty($i) check
	je .L3
	movq 0x10(%r14), %rax            ; return 2
	...
.L3:                                     ; return 1
	...