mesa: fix double->float assignment warnings, int/uint comparison warnings

Reported by Karl Schultz.
diff --git a/src/mesa/shader/prog_execute.c b/src/mesa/shader/prog_execute.c
index c212790..5641014 100644
--- a/src/mesa/shader/prog_execute.c
+++ b/src/mesa/shader/prog_execute.c
@@ -1017,12 +1017,12 @@
             /* XXX we could probably just use pow() here */
             if (a[0] > 0.0F) {
                if (a[1] == 0.0 && a[3] == 0.0)
-                  result[2] = 1.0;
+                  result[2] = 1.0F;
                else
                   result[2] = (GLfloat) _mesa_pow(a[1], a[3]);
             }
             else {
-               result[2] = 0.0;
+               result[2] = 0.0F;
             }
             result[3] = 1.0F;
             store_vector4(inst, machine, result);