fp_round's produced by getCopyFromParts should always be exact, because

they are produced by calls (which are known exact) and by cross block copies
which are known to be produced by extends.

This improves:

define double @test2() {
	%tmp85 = call double asm sideeffect "fld0", "={st(0)}"()
	ret double %tmp85
}

from:

_test2:
	subl	$20, %esp
	# InlineAsm Start
	fld0
	# InlineAsm End
	fstpl	8(%esp)
	movsd	8(%esp), %xmm0
	movsd	%xmm0, (%esp)
	fldl	(%esp)
	addl	$20, %esp
	#FP_REG_KILL
	ret

to:

_test2:
	# InlineAsm Start
	fld0
	# InlineAsm End
	#FP_REG_KILL
	ret

by avoiding a f64 <-> f80 trip

llvm-svn: 48108