fix a bogus assertion exposed by a recent change: packing the

fix a bogus assertion exposed by a recent change: packing the
struct may cause it to shrink more than one byte.  Before
my recent changes we compiled the new test into:

%0 = type { [6 x i8] }
@x = global %0 { [6 x i8] undef }, align 2        ; <%0*> [#uses=0]

which is obviously bogus.  Now we compile it into:

%0 = type <{ i32, i8, i8 }>
@x = global %0 zeroinitializer, align 2           ; <%0*> [#uses=0]

Where the last byte only is tail padding.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@101536 91177308-0d34-0410-b5e6-96231b3b80d8