X86: Fix argument register indexing.

X86: Fix argument register indexing.
Code was assuming that all argument registers followed in order from ArgumentReg0. There is now an ArgumentReg array which is indexed to find the right index. There is a constant, NumArgumentRegs, which can be used to protect against using an invalid ArgumentReg.

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extra : convert_revision : f448a3ca4d6adc3fc3323562870f70eec05a8a1f