fix(emit): PHI predecessor for and/or operand that emits sub-CFG (issue 0078)

A string `==`/`!=` used as an operand of a short-circuit `and`/`or` emitted
invalid LLVM (`PHI node entries do not match predecessors!`). String compares
expand into their own memcmp sub-CFG during LLVM emission, so the operand
finishes in a later basic block (`str.merge`) than the one the IR block
started in. `fixupPhiNodes` wired the short-circuit merge PHI's incoming edge
to `block_map[ir_block]` (the block the IR block started as), recording a
stale predecessor (`%entry`/`%and.rhs.0`).

Fix: record the builder's actual insertion block after emitting each IR
block's instructions (`term_block_map`, via `LLVMGetInsertBlock`) and use it
as the PHI predecessor. General — corrects the incoming block for any operand
that emitted intermediate basic blocks (string `==`, value `match`, …), not
just string `==`.

Regression: examples/0045-basic-string-eq-short-circuit.sx (string `==` on
both sides of `and` and of `or`, plus a match-value + enum-payload `==` shape).
Fails (LLVM abort) pre-fix, passes after.

examples/expected/0045-basic-string-eq-short-circuit.exit

@@ -0,0 +1 @@
+0

examples/expected/0045-basic-string-eq-short-circuit.stderr

@@ -0,0 +1 @@
+

examples/expected/0045-basic-string-eq-short-circuit.stdout

@@ -0,0 +1,5 @@
+and: true false false
+or: false true true
+mixed: true
+ne: true
+payload: true false