[CodeGen]: don't treat structures returned in registers as memory inputs

Summary:
The "=r" output constraint for a structure variable passed to inline asm
shouldn't be converted to "=*r", as this changes the asm directive
semantics and prevents DSE optimizations.
Instead, preserve the constraints and return such structures as integers
of corresponding size, which are converted back to structures when
storing the result.

Fixes PR42672.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65234

llvm-svn: 370335