When a bit-slice loads a value whose destination type is wider than the bit-slice itself, and the destination type is a signed integer, the loaded value must be sign-extended. The load accumulator always begins as the zero pattern, and the loaders do not attempt to detect a sign bit before they begin.
As such, this function takes a value loaded out of a bit-slice, which has been zero-extended from the storage length to the destination type, and the length of the bit-slice that contained it. If the destination type is unsigned, then the value is returned as-is; if the destination type is signed, then the value is sign-extended according to the bit at 1 << (width - 1).
I: The integer type of the loaded element. When this is one of u{8,16,32,64,size}, no sign extension takes place.elem: The value loaded out of a bit-slice.width: The width in bits of the source bit-slice. This is always known to be in the domain 1 ..= I::BITS.A correctly-signed copy of elem. Unsigned integers, and signed integers whose most significant loaded bit was 0, are untouched. Signed integers whose most significant loaded bit was 1 have their remaining high bits set to 1 for sign extension.