GS-hw: Fix recent alpha max check.

[lightningterror]

Description of Changes

GS: Redo GetAlphaMinMax.
Cleaner this way, also fix recent regression on Full Blend with alpha max.

GS-hw: Move the blend mix alpha check after sw blending selection.
Optimization.

Rationale behind Changes

Fix recent regression and optimization.

Suggested Testing Steps

Test blending and make sure still works on both renderers.

[TellowKrinkle]

Instead of this mess where we try to do all sorts of checks to only run GetAlphaMinMax() when needed, why not just have GetAlphaMinMax() return a reference to m_vt.alpha and then just do GetAlphaMinMax().max

Then we won't accidentally mess up conditions and we won't have to worry about that anymore at all

[TellowKrinkle]

Side note, GetAlphaMinMax() already checks m_vt.m_alpha.valid for an early return. But if you're worried about function call overhead, since compilers are terrible at splitting functions into a small inlinable part and a big rest of the function, you can do that manually and split GetAlphaMinMax() into

private:
	void CalcAlphaMinMax();
public:
	GSVertexTrace::VertexAlpha& GetAlphaMinMax()
	{
		if (!m_vt.m_alpha.valid)
			CalcAlphaMinMax();
		return m_vt.m_alpha;
	}