[Sparc] Complex return value ABI compliance.

According to the Sparc V8 ABI, complex numbers should be passed and returned as pairs of registers:

https://docs.oracle.com/cd/E26502_01/html/E28387/gentextid-2734.html

This fix ensures this is the case. Without this, complex numbers are returned as a struct of two floats, which breaks the ABI rules.

Differential Review: http://reviews.llvm.org/D20955

llvm-svn: 272148