[lang] Broken ti.math.sign implementations

[bobcao3]

import taichi as ti

@ti.kernel
def test(val : ti.f32) -> ti.f32:
    return ti.math.sign(val)

ti.init(arch=ti.gpu)

print(test(0.5))
print(test(-0.5))
print(test(0.0))

Under LLVM backends, prints:

1.0
1.0
0.0

Under GFX backends, crash with

Assertion failed: type->AsInteger(), file C:\Users\bobca\source\repos\taichi\external\SPIRV-Tools\source\opt\folding_rules.cpp, line 86

[ailzhang]

Took a quick look that it seems to be related to we feed a u1 to somewhere requires a 32bits int. Transferring to @listerily to send a fix, thanks!

[ailzhang]

Closing via #8082, thanks @listerily !