implement InstCombine/shift-trunc-shift.ll. This allows

implement InstCombine/shift-trunc-shift.ll.  This allows
us to compile:
#include <math.h>
int t1(double d) { return signbit(d); }

into:

_t1:
	movd	%xmm0, %rax
	shrq	$63, %rax
	ret

instead of:

_t1:
	movd	%xmm0, %rax
	shrq	$32, %rax
	shrl	$31, %eax
	ret

on x86-64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45311 91177308-0d34-0410-b5e6-96231b3b80d8