Pull in r175360 from upstream llvm trunk:

MCParser: Reject .balign with non-pow2 alignments.

  GNU as rejects them and there are configure scripts in the wild that
  check if the assembler rejects ".align 3" to determine whether the
  alignment is in bytes or powers of two.

MFC after:	3 days

contrib/llvm/lib/MC/MCParser/AsmParser.cpp

@@ -2372,6 +2372,10 @@ bool AsmParser::ParseDirectiveAlign(bool IsPow2, unsigned ValueSize) {
     }
 
     Alignment = 1ULL << Alignment;
+  } else {
+    // Reject alignments that aren't a power of two, for gas compatibility.
+    if (!isPowerOf2_64(Alignment))
+      Error(AlignmentLoc, "alignment must be a power of 2");
   }
 
   // Diagnose non-sensical max bytes to align.