stage1: saturating shl operates using LHS type

Saturating shift left (`<<|`) previously used the `ir_analyze_bin_op_math`
codepath rather than the `ir_analyze_bit_shift` codepath, leading to it
doing peer type resolution (incorrect) instead of using the LHS type as
the number of bits to do the saturating against.

This required implementing SIMD vector support for `@truncate`.

Additionall, this commit adds a compile error for saturating shift left
on a comptime_int.

stage2 does not pass these new behavior tests yet.

closes #10298

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