[CIR] Allow multi-block ctor regions on GlobalOp (#193596)
C++14 variable templates with non-constexpr constructors (e.g., `static
const Foo<N> x{}` where `Foo()` is not constexpr) crash CIR codegen
with:
```
'cir.global' op region #0 ('ctorRegion') failed to verify
constraint: region with at most 1 blocks
```
The problem is that `GlobalOp`'s `ctorRegion` is declared as
`MaxSizedRegion<1>`, but when exceptions are enabled, `emitAggExpr` can
create additional blocks in the ctor region for EH cleanup scaffolding
(unreachable/trap terminators). These extra blocks are dead code —
`LoweringPrepare` already discards them when it moves the ctor region
into `__cxx_global_var_init`.
This patch relaxes the constraint to `AnyRegion` and replaces the
`llvm_unreachable("Multiple blocks NYI")` in the static-local guard path
with an assert (that path will be reworked by #193576).
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