[AArch64] Make assert messages uniform and general [NFC]

Summary: Make assert messages related to Darwin, ELF and COFF uniform.

Reviewers: rnk, ruiu, compnerd, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, aemerson, rengolin, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D34730

llvm-svn: 306589
diff --git a/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp b/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
index 2bdc053..f82b9db 100644
--- a/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
+++ b/llvm/lib/Target/AArch64/AArch64MCInstLower.cpp
@@ -168,8 +168,7 @@
   if (Printer.TM.getTargetTriple().isOSBinFormatCOFF())
     return lowerSymbolOperandCOFF(MO, Sym);
 
-  assert(Printer.TM.getTargetTriple().isOSBinFormatELF() &&
-         "Expect Darwin, ELF or COFF target");
+  assert(Printer.TM.getTargetTriple().isOSBinFormatELF() && "Invalid target");
   return lowerSymbolOperandELF(MO, Sym);
 }