Implement some dag combines that allow doing fneg/fabs/fcopysign in integer

Implement some dag combines that allow doing fneg/fabs/fcopysign in integer
registers if used by a bitconvert or using a bitconvert.  This allows us to
avoid constant pool loads and use cheaper integer instructions when the
values come from or end up in integer regs anyway.  For example, we now
compile CodeGen/X86/fp-in-intregs.ll to:

_test1:
	movl	$2147483648, %eax
	xorl	4(%esp), %eax
	ret
_test2:
	movl	$1065353216, %eax
	orl	4(%esp), %eax
	andl	$3212836864, %eax
	ret

Instead of:
_test1:
	movss	4(%esp), %xmm0
	xorps	LCPI2_0, %xmm0
	movd	%xmm0, %eax
	ret
_test2:
	movss	4(%esp), %xmm0
	andps	LCPI3_0, %xmm0
	movss	LCPI3_1, %xmm1
	andps	LCPI3_2, %xmm1
	orps	%xmm0, %xmm1
	movd	%xmm1, %eax
	ret

bitconverts can happen due to various calling conventions that require
fp values to passed in integer regs in some cases, e.g. when returning
a complex.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46414 91177308-0d34-0410-b5e6-96231b3b80d8