[AMDGPU] Improve error checking in gfx10 assembler tests

[foad]

Summary

This adds checks that the expected error was actually reported against
the correct instruction, and fixes a couple of problems that that showed
up: one incorrect W32-ERR:

v_cmp_class_f16_sdwa vcc, v1, v2 src0_sel:DWORD src1_sel:DWORD
W64: encoding: [0xf9,0x04,0x1e,0x7d,0x01,0x00,0x06,0x06]
- W32-ERR: error: invalid operand for instruction
+// W32-ERR: error: {{instruction not supported on this GPU|invalid operand for instruction}}

and one missing W32-ERR:

v_cmp_class_f16_sdwa s[6:7], v1, v2 src0_sel:DWORD src1_sel:DWORD
W64: encoding: [0xf9,0x04,0x1e,0x7d,0x01,0x86,0x06,0x06]
+ W32-ERR: error: invalid operand for instruction

[merge_guards_bot]

Unit tests: pass. 61782 tests passed, 0 failed and 781 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[rampitec]

LGTM