Fix pattern match failure in decodeM for ConstantStruct of NamedTypeReference type.

example:
  %struct.ps = type { i32, i32 }
  @fib.p = private unnamed_addr constant %struct.ps { i32 0, i32 1 }, align 4
Merge pull request #81 from lambdageek/llvm-3.3

Fix pattern match failure in decodeM for ConstantStruct of NamedTypeReference type. (The decoded type of a named struct doesn't directly have an isPacked field).