Fix two unreachable-guard style bugs in GPU state and shader gen

ReplaceAlphaWithStencil's blue-to-alpha check sat after a branch that already
catches every replaceBlend except NO and READ_FRAMEBUFFER - so it was dead, and
blue-to-alpha draws could come back REPLACE_ALPHA_YES or DUALSOURCE and pick up
stencil-to-alpha on top of the blue-to-alpha setup. Moved it above.

The D3D11 fragment shader applied nointerpolation to v_color0 but not v_color1,
and in HLSL it's the PS input declaration that decides - so flat shading was lost
for the specular term. The vertex shader already marks both.

GPU/Common/FragmentShaderGenerator.cpp

@@ -286,7 +286,10 @@ bool GenerateFragmentShader(const FShaderID &id, char *buffer, const ShaderLangu
 		const char *colorInterpolation = doFlatShading && compat.shaderLanguage == HLSL_D3D11 ? "nointerpolation " : "";
 		WRITE(p, "  %svec4 v_color0: COLOR0;\n", colorInterpolation);
 		if (lmode) {
-			WRITE(p, "  vec3 v_color1: COLOR1;\n");
+			// Needs the same nointerpolation as v_color0 - in HLSL it's the PS input declaration
+			// that decides, so without it the specular term keeps interpolating when flat shading
+			// is on. Both GLSL paths already mark both.
+			WRITE(p, "  %svec3 v_color1: COLOR1;\n", colorInterpolation);
 		}
 		WRITE(p, "  float v_fogdepth: TEXCOORD1;\n");
 		if (needFragCoord) {

GPU/Common/GPUStateUtils.cpp

@@ -185,6 +185,10 @@ ReplaceAlphaType ReplaceAlphaWithStencil(ReplaceBlendType replaceBlend) {
 		return REPLACE_ALPHA_NO;
 	}
 
+	if (replaceBlend == ReplaceBlendType::REPLACE_BLEND_BLUE_TO_ALPHA) {
+		return REPLACE_ALPHA_NO;  // irrelevant
+	}
+
 	if (replaceBlend != REPLACE_BLEND_NO && replaceBlend != REPLACE_BLEND_READ_FRAMEBUFFER) {
 		if (nonAlphaSrcFactors[gstate.getBlendFuncA()] && nonAlphaDestFactors[gstate.getBlendFuncB()]) {
 			return REPLACE_ALPHA_YES;
@@ -197,10 +201,6 @@ ReplaceAlphaType ReplaceAlphaWithStencil(ReplaceBlendType replaceBlend) {
 		}
 	}
 
-	if (replaceBlend == ReplaceBlendType::REPLACE_BLEND_BLUE_TO_ALPHA) {
-		return REPLACE_ALPHA_NO;  // irrelevant
-	}
-
 	return REPLACE_ALPHA_YES;
 }