[AMDGPU] Improve error checking in gfx10 assembler tests

[AMDGPU] Improve error checking in gfx10 assembler tests

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

Reviewers: rampitec, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72611