arch-vega: Validate if scalar sources are scalar gprs
arch-vega: Validate if scalar sources are scalar gprs

Port the fixes for scalar source checks from arch-gcn3 at
https://gem5-review.googlesource.com/c/public/gem5/+/48344.

Scalar sources can either be a general-purpose register or a constant
register that holds a single value.

If we don't check for if the register is a general-purpose register,
it's possible that we get a constant register, which then causes all of
the register mapping code to break, as the constant registers aren't
supposed to be mapped like the general-purpose registers are.

This fix adds an isScalarReg check to the instruction encodings that
were missing it.

Change-Id: I30dd2d082a5a1dcc3075843bcefd325113ed1df6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55343
Reviewed-by: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Maintainer: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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