[InstCombine] decomposeSimpleLinearExpr should bail out on negative operands.

InstCombine tries to rewrite

  %prod = mul nsw i64 %X,   Scale
  %acc = add nsw i64 %prod,   Offset
  %0 = alloca i8, i64 %acc, align 4
  %1 = bitcast i8* %0 to i32*
  Use ( %1 )

into

  %prod = mul nsw i64 %X,   Scale/4
  %acc = add nsw i64 %prod,   Offset/4
  %0 = alloca i32, i64 %acc, align 4
  Use (%0)

But it assumes Scale is unsigned, and performs an unsigned division.
So we should bail out if Scale cannot be interpreted as an unsigned safely.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D126546