fcvt_from_uint for 8bit and 16bit uints is incorrect

[bjorn3]

v0 = iconst.i8 0xff
v1 = fcvt_from_uint.f32 v0

Gives 254.0 instead of 255.0.

[bjorn3]

This breaks the saturating-float-casts test of the rustc test suite.

cc bjorn3/rustc_codegen_cranelift#381

[bjorn3]

Minimal repro:

test compile
target x86_64

function u0:0() system_v {
ebb0:
    v0 = iconst.i8 255
    v1 = fcvt_from_uint.f32 v0

    v5 = f32const 0x1.fe0000p7
    v7 = fcmp ne v1, v5
    brnz v7, ebb1
    return

ebb1:
    trap user65535
}

[bjorn3]

It seems the generic legalization with uextend of input operand doesn't get triggered, but it immediately uses the x86 legalization, which doesn't uextend first.