stb_vorbis: Change imdct_step3_inner_s_loop_ld654

Released Clang 12 generates bad code for the original loop in here.
While this is a compiler bug plain and simple, we still have to deal
with it.

This is related to the SLP vectorizer, and in particular the two
reverse subtracts in the butterflies for the second half to avoid
unary negates.

Use the more regular dataflow that has the unary negates in it
(we can at least fold one of them into a constant, namely for A2)
and introduce a few temporaries that also make alias analysis (and
possible block-level vectorization) a whole let easier while I'm at
it.

This fixes the codegen issues on Clang 12, which now produces a
working decoder, and I expect the single unary negate that we
actually gain per iteration of this loop is not a significant
perf concern. (There are bigger fish to fry here regardless.)

Fixes issue #1152.

stb_vorbis.c

@@ -2592,34 +2592,33 @@ static void imdct_step3_inner_s_loop_ld654(int n, float *e, int i_off, float *A,
 
    while (z > base) {
       float k00,k11;
-
-      k00   = z[-0] - z[-8];
-      k11   = z[-1] - z[-9];
-      z[-0] = z[-0] + z[-8];
-      z[-1] = z[-1] + z[-9];
-      z[-8] =  k00;
-      z[-9] =  k11 ;
-
-      k00    = z[ -2] - z[-10];
-      k11    = z[ -3] - z[-11];
-      z[ -2] = z[ -2] + z[-10];
-      z[ -3] = z[ -3] + z[-11];
-      z[-10] = (k00+k11) * A2;
-      z[-11] = (k11-k00) * A2;
-
-      k00    = z[-12] - z[ -4];  // reverse to avoid a unary negation
+      float l00,l11;
+
+      k00    = z[-0] - z[ -8];
+      k11    = z[-1] - z[ -9];
+      l00    = z[-2] - z[-10];
+      l11    = z[-3] - z[-11];
+      z[ -0] = z[-0] + z[ -8];
+      z[ -1] = z[-1] + z[ -9];
+      z[ -2] = z[-2] + z[-10];
+      z[ -3] = z[-3] + z[-11];
+      z[ -8] = k00;
+      z[ -9] = k11;
+      z[-10] = (l00+l11) * A2;
+      z[-11] = (l11-l00) * A2;
+
+      k00    = z[ -4] - z[-12];
       k11    = z[ -5] - z[-13];
+      l00    = z[ -6] - z[-14];
+      l11    = z[ -7] - z[-15];
       z[ -4] = z[ -4] + z[-12];
       z[ -5] = z[ -5] + z[-13];
-      z[-12] = k11;
-      z[-13] = k00;
-
-      k00    = z[-14] - z[ -6];  // reverse to avoid a unary negation
-      k11    = z[ -7] - z[-15];
       z[ -6] = z[ -6] + z[-14];
       z[ -7] = z[ -7] + z[-15];
-      z[-14] = (k00+k11) * A2;
-      z[-15] = (k00-k11) * A2;
+      z[-12] = k11;
+      z[-13] = -k00;
+      z[-14] = (l11-l00) * A2;
+      z[-15] = (l00+l11) * -A2;
 
       iter_54(z);
       iter_54(z-8);