PixelShaderGen: Cleanup some tev code.

BPMemory: Assign a more descriptive name to a field in the genmode register.
PixelShaderGen: Declare variables as late as possible (or not at all if it's not necessary).
PixelShaderGen: Optimize tev stage input loading
PixelShaderGen: Use correct values for constants other than one and zero.
PixelShaderGen: Fix alphabump value not being of the correct precision.
Move some tables to wherever they're used.
PixelShaderGen: More precision fixes for normalized bump alpha and tevind offset.
PixelShaderGen: Cleanup SampleTexture calls.
PixelShaderGen: Cleanups + another precision fix.
PixelShaderGen: Further cleanups.
PixelShaderGen: Fix a shader compile error introduced in revision 4968cdf86bb5.