[RISCV][AArch64][IRGen] Add a special case to CodeGenFunction::EmitCall for...

[RISCV][AArch64][IRGen] Add a special case to CodeGenFunction::EmitCall for scalable vector return being coerced to fixed vector.

Before falling back to CreateCoercedStore, detect a scalable vector
return being coerced to fixed vector. Handle it using a vector.extract
intrinsic without going through memory.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D155495