Fixed a bug with how non power 2 types were written to memory

[kren1]

This is meant to fix a bug which manifests itself in the following two snippets:

union U0 {
    signed f3 :18;
};

static union U0 g_988 = { 0UL };

int main(int argc, char* argv[]) {
    g_988.f3 = 534;
    printf("f3 %d \n", g_988.f3);
    return 0;
}

This one works on the current master if pragma pack(1) is removed:

#include <stdio.h>
#pragma pack(1)
struct S0 {
    signed f0 :20;
};

static struct S0 x;
int main(int argc, char* argv[]) {

    x = (struct S0 ) { .f0 = 32 };
    printf("integer %d\n", x.f0);
    return x.f0;
}

What happens is that in these cases w = 24, which makes it jump to Int8 case in the switch(w), which causes the wrong value to be stored. I've added a check to make sure that w is a power of 2, so it definitively fits into one of the switch cases and uses the slow version in all the others.

I think this is a better option than adding Int24 into the switch since then all the others are also needed.

[ccadar]

Thanks, @kren1. Just a comment to document that this happens only with LLVM 3.4, not 2.9.

[delcypher]

@ccadar Is there any reason not to merge this?

[delcypher]

I've manually merged this commit with a minor tweak.