SPV: RelaxedPrecision: use the result precision for texture sampling.

Fix #2298.

The AST has two precisions, an operation precision and a result precision.
Actual use of GLSL with mediump samplers wants the result precision, so
pick that up instead of the operation precision.
diff --git a/SPIRV/GlslangToSpv.cpp b/SPIRV/GlslangToSpv.cpp
index e0480d1..295660b 100644
--- a/SPIRV/GlslangToSpv.cpp
+++ b/SPIRV/GlslangToSpv.cpp
@@ -4730,7 +4730,7 @@
         translateArguments(*node->getAsAggregate(), arguments, lvalueCoherentFlags);
     else
         translateArguments(*node->getAsUnaryNode(), arguments);
-    spv::Decoration precision = TranslatePrecisionDecoration(node->getOperationPrecision());
+    spv::Decoration precision = TranslatePrecisionDecoration(node->getType());
 
     spv::Builder::TextureParameters params = { };
     params.sampler = arguments[0];