GLSL: Fix #1833: Don't constant fold integer mix to a float.

Test/310.frag

@@ -58,8 +58,8 @@ void foo23()
     b1 = mix(b2, b3, b);
     uvec3 um3 = mix(uvec3(i), uvec3(i), bvec3(b));
     ivec4 im4 = mix(ivec4(i), ivec4(i), bvec4(b));
+    1 << mix(1u, 1u, false);  // does not require folding
 }
-
 layout(binding=3) uniform sampler2D s1;
 layout(binding=3) uniform sampler2D s2; // ERROR: overlapping bindings?  Don't see that in the 310 spec.
 highp layout(binding=2) uniform writeonly image2D      i2D;

Test/baseResults/310.frag.out

@@ -337,6 +337,16 @@ ERROR: node is still EOpNull!
 0:60              'i' ( uniform mediump int)
 0:60            Construct bvec4 ( temp 4-component vector of bool)
 0:60              'b' ( temp bool)
+0:61      left-shift ( temp int)
+0:61        Constant:
+0:61          1 (const int)
+0:61        mix ( global uint)
+0:61          Constant:
+0:61            1 (const uint)
+0:61          Constant:
+0:61            1 (const uint)
+0:61          Constant:
+0:61            false (const bool)
 0:98  Function Definition: foots( ( global void)
 0:98    Function Parameters: 
 0:100    Sequence