On X86, casting an integer to floating point requires going through memory.

If the source of the cast is a load, we can just use the source memory location,
without having to create a temporary stack slot entry.

Before we code generated this:

double %int(int* %P) {
        %V = load int* %P
        %V2 = cast int %V to double
        ret double %V2
}

into:

int:
        sub %ESP, 4
        mov %EAX, DWORD PTR [%ESP + 8]
        mov %EAX, DWORD PTR [%EAX]
        mov DWORD PTR [%ESP], %EAX
        fild DWORD PTR [%ESP]
        add %ESP, 4
        ret

Now we produce this:

int:
        mov %EAX, DWORD PTR [%ESP + 4]
        fild DWORD PTR [%EAX]
        ret

... which is nicer.

llvm-svn: 12846