[BUG] Count leading zeros can't be used at compile time

[martinvuyk]

Bug description

Count leading zeros can't be used at compile time. This is actually part of #933. This blocks a lot of other work to fully support unicode due to it needing this function.

part of the error: note: LLVM intrinsic operand has unknown value: #pop<simd 15> : !pop.scalar<ui8>

CC: @lattner

Steps to reproduce

from bit import count_leading_zeros

fn _utf8_first_byte_sequence_length(b: Byte) -> Int:
    """Get the length of the sequence starting with given byte. Do note that
    this does not work correctly if given a continuation byte."""

    debug_assert(
        (b & 0b1100_0000) != 0b1000_0000,
        "Function does not work correctly if given a continuation byte.",
    )
    return int(count_leading_zeros(~b)) + int(b < 0b1000_0000)

fn main():
    print(_utf8_first_byte_sequence_length(ord("a")) == 1)
    print(_utf8_first_byte_sequence_length("🔥".as_bytes()[0]) == 4)
    alias `🔥` = "🔥".as_bytes()[0]
    alias length = _utf8_first_byte_sequence_length(`🔥`)
    print(length == 4)

System information

- Provide version information for Mojo by pasting the output of `mojo -v`
`mojo 24.6.0.dev2024120805`