[AMDGPU] Use @LINE for error checking in gfx10 assembler tests

Summary:
This is a rework of D72611, using @LINE to check that errors are
reported against the right instruction instead of adding lots of extra
*-ERR-NEXT: check lines.

Reviewers: rampitec, arsenm, nhaehnle

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74227