Teach instcombine 4 new xforms:

  (add (sext x), cst) --> (sext (add x, cst'))
  (add (sext x), (sext y)) --> (sext (add int x, y))
  (add double (sitofp x), fpcst) --> (sitofp (add int x, intcst))
  (add double (sitofp x), (sitofp y)) --> (sitofp (add int x, y))

This generally reduces conversions.  For example MiBench/telecomm-gsm
gets these simplifications:

HACK2: 	%tmp67.i142.i.i = sext i16 %tmp6.i141.i.i to i32		; <i32> [#uses=1]
	%tmp23.i139.i.i = sext i16 %tmp2.i138.i.i to i32		; <i32> [#uses=1]
	%tmp8.i143.i.i = add i32 %tmp67.i142.i.i, %tmp23.i139.i.i		; <i32> [#uses=3]
HACK2: 	%tmp67.i121.i.i = sext i16 %tmp6.i120.i.i to i32		; <i32> [#uses=1]
	%tmp23.i118.i.i = sext i16 %tmp2.i117.i.i to i32		; <i32> [#uses=1]
	%tmp8.i122.i.i = add i32 %tmp67.i121.i.i, %tmp23.i118.i.i		; <i32> [#uses=3]
HACK2: 	%tmp67.i.i190.i = sext i16 %tmp6.i.i189.i to i32		; <i32> [#uses=1]
	%tmp23.i.i187.i = sext i16 %tmp2.i.i186.i to i32		; <i32> [#uses=1]
	%tmp8.i.i191.i = add i32 %tmp67.i.i190.i, %tmp23.i.i187.i		; <i32> [#uses=3]
HACK2: 	%tmp67.i173.i.i.i = sext i16 %tmp6.i172.i.i.i to i32		; <i32> [#uses=1]
	%tmp23.i170.i.i.i = sext i16 %tmp2.i169.i.i.i to i32		; <i32> [#uses=1]
	%tmp8.i174.i.i.i = add i32 %tmp67.i173.i.i.i, %tmp23.i170.i.i.i		; <i32> [#uses=3]
HACK2: 	%tmp67.i152.i.i.i = sext i16 %tmp6.i151.i.i.i to i32		; <i32> [#uses=1]
	%tmp23.i149.i.i.i = sext i16 %tmp2.i148.i.i.i to i32		; <i32> [#uses=1]
	%tmp8.i153.i.i.i = add i32 %tmp67.i152.i.i.i, %tmp23.i149.i.i.i		; <i32> [#uses=3]
HACK2: 	%tmp67.i.i.i.i = sext i16 %tmp6.i.i.i.i to i32		; <i32> [#uses=1]
	%tmp23.i.i5.i.i = sext i16 %tmp2.i.i.i.i to i32		; <i32> [#uses=1]
	%tmp8.i.i7.i.i = add i32 %tmp67.i.i.i.i, %tmp23.i.i5.i.i		; <i32> [#uses=3]


This also fixes a bug in ComputeNumSignBits handling select and
makes it more aggressive with and/or.

llvm-svn: 51302