[SCEV] Compute SCEV for ashr(add(shl(x, n), c), m) instr triplet

%x = shl i64 %w, n
%y = add i64 %x, c
%z = ashr i64 %y, m

The above given instruction triplet is seen many times in the generated
LLVM IR, but SCEV model is not able to compute the SCEV value of AShr
instruction in this case.

This patch models the two cases of the above instruction pattern using
the following expression:

=> sext(add(mul(trunc(w), 2^(n-m)), c >> m))

1) when n = m the expression reduces to sext(add(trunc(w), c >> n))
as n-m=0, and multiplying with 2^0 gives the same result.

2) when n > m the expression works as given above.

It also adds several unittest to verify that SCEV is able to compute
the value.

$ opt sext-add-inreg.ll -passes="print<scalar-evolution>"

Comparing the snippets of the result of SCEV analysis:

* SCEV of ashr before change
----------------------------
%idxprom = ashr exact i64 %sext, 32
  -->  %idxprom U: [-2147483648,2147483648) S: [-2147483648,2147483648)
  Exits: 8                LoopDispositions: { %for.body: Variant }

* SCEV of ashr after change
---------------------------
%idxprom = ashr exact i64 %sext, 32
  -->  {0,+,1}<nuw><nsw><%for.body> U: [0,9) S: [0,9)
  Exits: 8                LoopDispositions: { %for.body: Computable }

LoopDisposition of the given SCEV was LoopVariant before, after adding
the new way to model the instruction, the LoopDisposition becomes
LoopComputable as it is able to compute the SCEV of the instruction.

Differential Revision: https://reviews.llvm.org/D152278