Improve LOAD_CONST codegen

This is a clean-up of Sean McQuillan's patch at http://codereview.appspot.com/74058.

Performance:

$ tests/perf.py -r --args "-j always -O2" -b ai,django,slowspitfire,slowpickle,slowunpickle control/python trunk3/python 

ai (n_queens only):
Min: 0.441 -> 0.432: 2.07% faster
Avg: 0.443 -> 0.435: 2.04% faster
Significant (t=28.349679, a=0.95)

django:
Min: 1.053 -> 1.064: 0.99% slower
Avg: 1.056 -> 1.068: 1.13% slower
Significant (t=-33.003287, a=0.95)

slowpickle:
Min: 0.640 -> 0.646: 1.03% slower
Avg: 0.641 -> 0.648: 1.13% slower
Significant (t=-28.428479, a=0.95)

slowspitfire:
Min: 0.672 -> 0.672: 0.04% slower
Avg: 0.673 -> 0.674: 0.12% slower
Significant (t=-3.018280, a=0.95)

slowunpickle:
Min: 0.292 -> 0.284: 2.87% faster
Avg: 0.292 -> 0.284: 2.81% faster
Significant (t=60.641397, a=0.95)


LLVM IR code change for "def foo(): return 7"

## AFTER ##

line_start:		; preds = %continue_entry
	%50 = load i32* getelementptr (%struct._object* inttoptr (i64 145752472 to %struct._object*), i32 0, i32 0), align 8		; <i32> [#uses=1]
	%51 = add i32 %50, 1		; <i32> [#uses=1]
	store i32 %51, i32* inttoptr (i32 145752472 to i32*), align 8
	store %struct._object* inttoptr (i64 145752472 to %struct._object*), %struct._object** %stack_bottom
	%f_iblock = getelementptr %struct._frame* %frame, i32 0, i32 19		; <i8*> [#uses=1]
	br label %unwind_loop_header


## BEFORE ##	
	
line_start:		; preds = %continue_entry
	%53 = getelementptr %struct._object* %3, i32 2		; <%struct._object*> [#uses=1]
	%54 = bitcast %struct._object* %53 to %struct._object**		; <%struct._object**> [#uses=1]
	%55 = load %struct._object** %54		; <%struct._object*> [#uses=9]
	%56 = getelementptr %struct._object* %55, i32 0, i32 0		; <i32*> [#uses=4]
	%57 = load i32* %56		; <i32> [#uses=1]
	%58 = add i32 %57, 1		; <i32> [#uses=1]
	store i32 %58, i32* %56
	store %struct._object* %55, %struct._object** %stack_bottom
	%f_iblock = getelementptr %struct._frame* %frame, i32 0, i32 19		; <i8*> [#uses=1]
	br label %unwind_loop_header

[Jeffrey Yasskin, 2009-06-17 15:57:19]

LGTM