JIT: Allow strength reducing to GCD of IVs

[jakobbotsch]

This adds support for strength reduction to create a new primary IV that is the GCD of several IVs found in the loop. When the same index is used to access arrays of different sizes we will often see the IV being multiplied by different values; however, it is usually still profitable to strength reduce to the GCD of the step values and then "recover" the final IV by scaling.

Example:

public static void Foo()
{
    string puzzle = "003020600900305001001806400008102900700000008006708200002609500800203009005010300";
    int[] board = new int[81];

    for (int i = 0; i < puzzle.Length; i++)
    {
        board[i] = puzzle[i] - '0';
    }
}

Codegen diff for loop:

        xor      ecx, ecx
+       mov      edx, 81

 G_M24659_IG03:
-       mov      edx, ecx
-       movzx    r8, word  ptr [rbx+2*rdx+0x10]
+       movzx    r8, word  ptr [rbx+rcx+0x10]
        add      r8d, -48
-       mov      dword ptr [rax+4*rdx+0x10], r8d
-       inc      ecx
-       cmp      ecx, 81
-       jl       SHORT G_M24659_IG03
-						;; size=24 bbWeight=3.96 PerfScore 19.80
+       mov      dword ptr [rax+2*rcx+0x10], r8d
+       add      rcx, 2
+       dec      edx
+       jne      SHORT G_M24659_IG03
+						;; size=23 bbWeight=3.96 PerfScore 18.81

A similar diff in System.Linq.Enumerable+EnumerableSorter`2[System.__Canon,System.Decimal]:ComputeKeys(System.__Canon[],int):

+       xor      edx, edx
 G_M57524_IG05:
-       mov      edx, r15d
-       mov      r8, gword ptr [rbx+8*rdx+0x10]
+       mov      r8, gword ptr [rbx+rdx+0x10]
        vmovups  xmm0, xmmword ptr [r8+0x20]
        vmovups  xmmword ptr [rsp+0x28], xmm0
-       shl      rdx, 4
        vmovups  xmm0, xmmword ptr [rsp+0x28]
-       vmovups  xmmword ptr [r14+rdx+0x10], xmm0
-       inc      r15d
-       cmp      r13d, r15d
-       jg       SHORT G_M57524_IG05
-						;; size=45 bbWeight=75.73 PerfScore 1079.10
+       vmovups  xmmword ptr [r14+2*rdx+0x10], xmm0
+       add      rdx, 8
+       dec      r13d
+       jne      SHORT G_M57524_IG05
+						;; size=39 bbWeight=75.73 PerfScore 1022.31

Fix #102068
Fix #105241