Fix OpenCL pow and pown arguments

Spec fix appeared in SPIR-V 1.0 Rev 2:
https://www.khronos.org/registry/spir-v/specs/1.0/OpenCL.ExtendedInstructionSet.100.html#_changes_from_version_1_0_revision_1

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/143

CHANGES

@@ -17,3 +17,6 @@ v2016.0-dev 2016-04-22
      - Supports integer widths up to 64 bits
    - Validator is incomplete
    - Supported on Linux, OSX, Android, Windows
+
+ - Fixes bugs:
+   - #143: OpenCL pow and pown arguments

source/extinst-1.0.opencl.std.grammar.json

@@ -385,7 +385,6 @@
       "opname" : "pow",
       "opcode" : 48,
       "operands" : [
-        { "kind" : "IdRef" },
         { "kind" : "IdRef" },
         { "kind" : "IdRef" }
       ]
@@ -394,6 +393,7 @@
       "opname" : "pown",
       "opcode" : 49,
       "operands" : [
+        { "kind" : "IdRef" },
         { "kind" : "IdRef" }
       ]
     },

test/ExtInst.OpenCL.std.cpp

@@ -153,8 +153,8 @@ INSTANTIATE_TEST_CASE_P(
         CASE2(Modf, modf),
         CASE1(Nan, nan),
         CASE2(Nextafter, nextafter),
-        CASE3(Pow, pow),
-        CASE1(Pown, pown),
+        CASE2(Pow, pow),
+        CASE2(Pown, pown),
         CASE2(Powr, powr),
         CASE2(Remainder, remainder),
         CASE3(Remquo, remquo),