[Sparc] Get sret arg size from CallLoweringInfo.getArgs()

Summary:
Looking at the callee argument list, as is done now, might not work if
the function has been typecasted into one that is expected to return
a struct. This change also simplifies the code.

The isFP128ABICall() function can be removed as it is no longer needed.
The test in fp128.ll has been updated to verify this.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

Subscribers: fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D48117

llvm-svn: 340008