[stdlib] Improve type refinement for pack_bits

[soraros]

So the following works without rebind:

var v: UInt8 = pack_bits(SIMD[DType.bool, 8](1))

_uint gets inlined even when n isn't concrete, cluttering tooltips and types.
Can call-by-value be triggered only when n is concrete?

CC @lattner since it's related to @always_inline("builtin").

[soraros]

Using the if expression since hlcf.if is not supported in builtin.

[JoeLoser]

!sync

[modularbot]

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[modularbot]

Landed in a638dc7! Thank you for your contribution 🎉

[lattner]

Thanks @soraros . Just catching up: the point of always_inline(builtin) isn't to inline everything that "could be constant folded". It is really about things that are necessary to use in dependent types. There is longer term work to improve the Mojo intepreter that may make it more powerful - this will be useful for this sort of thing as well as for requires clauses - but the parser-time interpreter will always be limited to not knowing about the target (e.g. pointer sizes) so it will be inherently limited in what it can do.