[MC,test] Merge register-assignment-error.s into register-assignment.s and improve the test

To actually address my review comment in #75693

Author: MaskRay

llvm/test/MC/X86/register-assignment-error.s

@@ -1,4 +1,5 @@
 // RUN: llvm-mc -triple x86_64-unknown-unknown %s -o -      | FileCheck %s
+// RUN: not llvm-mc -triple x86_64 --defsym ERR=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR
 
 // CHECK-NOT: .set var_xdata
 var_xdata = %rcx
@@ -25,3 +26,12 @@ xorq var_xdata, var_xdata
 .else
   .byte 2
 .endif
+
+.ifdef ERR
+// ERR: [[#@LINE+1]]:5: error: expected absolute expression
+.if var_xdata == 1
+.endif
+// ERR: [[#@LINE+1]]:5: error: expected absolute expression
+.if 1 == var_xdata
+.endif
+.endif