mesa: fix instruction indexing bugs

We were looping over instructions but only looking at the 0th instruction's
opcode.

Fixes fd.o bug 27566.
diff --git a/src/mesa/shader/program.c b/src/mesa/shader/program.c
index f4f701b..f77a773 100644
--- a/src/mesa/shader/program.c
+++ b/src/mesa/shader/program.c
@@ -625,7 +625,7 @@
    GLuint i, j;
    for (i = 0; i < numInst; i++) {
       /* src regs */
-      for (j = 0; j < _mesa_num_inst_src_regs(inst->Opcode); j++) {
+      for (j = 0; j < _mesa_num_inst_src_regs(inst[i].Opcode); j++) {
          if (inst[i].SrcReg[j].File == oldFile &&
              inst[i].SrcReg[j].Index == oldIndex) {
             inst[i].SrcReg[j].File = newFile;
@@ -652,7 +652,7 @@
 {
    GLuint i, j;
    for (i = 0; i < numInst; i++) {
-      for (j = 0; j < _mesa_num_inst_src_regs(inst->Opcode); j++) {
+      for (j = 0; j < _mesa_num_inst_src_regs(inst[i].Opcode); j++) {
          GLuint f = inst[i].SrcReg[j].File;
          if (f == PROGRAM_CONSTANT ||
              f == PROGRAM_UNIFORM ||