HLSL: add .mips[][] operator for texture types

This implements mytex.mips[mip][coord] for texture types.  There is
some error testing, but not comprehensive.  The constructs can be
nested, e.g in this case the inner .mips is parsed before the completion
of the outer [][] operator.

   tx.mips[tx.mips[a][b].x][c]

gtests/Hlsl.FromFile.cpp

@@ -177,6 +177,9 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.logicalConvert.frag", "main"},
         {"hlsl.logical.unary.frag", "main"},
         {"hlsl.loopattr.frag", "main"},
+        {"hlsl.mip.operator.frag", "main"},
+        {"hlsl.mip.negative.frag", "main"},
+        {"hlsl.mip.negative2.frag", "main"},
         {"hlsl.namespace.frag", "main"},
         {"hlsl.nonint-index.frag", "main"},
         {"hlsl.matNx1.frag", "main"},