[InstCombine] reduce expanded form of integer square-of-sums

[rotateright]

; square-of-sum may be expanded in source/IR:
; x^2 + 2xy + y^2
; (x * x) + ((2 * x) + y) * y
define i6 @src(i6 %x, i6 %y) {
  %xx = mul i6 %x, %x
  %x2 = shl i6 %x, 1
  %x2y = add i6 %x2, %y
  %x2yy = mul i6 %x2y, %y
  %r = add i6 %x2yy, %xx
  ret i6 %r
}

; This can be reduced 
; (unless all of the intermediate values have extra uses?):
; (x + y)^2
define i6 @tgt(i6 %x, i6 %y) {
  %xy = add i6 %y, %x
  %r = mul i6 %xy, %xy
  ret i6 %r
}

https://alive2.llvm.org/ce/z/TkwEKJ

Note that no-wrap (nsw, nuw) can be propagated to the resulting math if (some subset of?) the original code has no-wrap. Also, there are probably 8 commuted variations of the last 3 math ops.

[rainerzufalldererste]

https://reviews.llvm.org/D156026
Doesn't have any tests yet, but does the InstCombine for int & float

[rainerzufalldererste]

Integer Version has been committed. Float: https://reviews.llvm.org/D158079