fix(0109): hoist all per-instruction allocas to the function entry block

An alloca built at its use site re-executes on every pass through that
block, and LLVM reclaims allocas only at ret — so loop-body locals,
nested-loop index slots, and emitter spill temps (ig.tmp, sret slots, ABI
coercion temps, byval materialization) grew the stack per iteration and
long loops segfaulted on stack exhaustion.

New LLVMEmitter.buildEntryAlloca inserts after existing entry-block
allocas and restores the builder position; every LLVMBuildAlloca site
reachable during instruction emission now routes through it.
Initialization stores stay at the use site (per-iteration re-init is
unchanged), and entry slots become mem2reg-promotable. The 35 .ir
snapshot diffs are pure alloca position moves (type multisets verified
identical per file).

Regression: examples/0047-basic-loop-local-stack-reuse.sx (segfaulted
pre-fix on both the 1M-iteration body-local loop and the 3M-iteration
nested loop).

examples/expected/0047-basic-loop-local-stack-reuse.stdout

@@ -0,0 +1,2 @@
+sum=499999500000
+n=3000000