[CIR] Pass x87 long double vectors on x86_64 (#230302)
Vectors of x87 long double now go through x86_64 calling-convention
lowering instead of hitting NYI. The ABI library sizes their elements at
128 bits like clang does, so the signatures match classic codegen.
Unions are still moved as a value of their storage type, and a long
double stores only 10 of its 16 bytes. So a union holding an x87 value
next to another member stays NYI unless it's a plain long double and the
other members fit in those 10 bytes. That also stops a silent miscompile
of unions like `union { long double ld; char c[16]; }`.
Assisted-by: Cursor / Claude Opus 5.5
