Fix genCodeForIndexAddr

[villelaitila]

This does some weird things - treats the array length as 64 bit when it's in fact 32 bit, fails to zero extend TYP_INT indices, creates new GT_IND/GT_LEA nodes out of thin air.

dotnet#20126 has a similar fix for ARM.

Sample generated code for a[i + 2]:

; before
       488B4510             mov      rax, gword ptr [rbp+10H]
       8B5518               mov      edx, dword ptr [rbp+18H]
       83C202               add      edx, 2
; 64 bit compare, typically works correctly due to the zero-extend to 64 bit model
; and because the array length field is followed by 4 bytes of padding that are
; normally 0
       483B5008             cmp      rdx, qword ptr [rax+8]
       730D                 jae      SHORT G_M55886_IG04
; use a TYP_INT value as 64 bit
       488D449010           lea      rax, bword ptr [rax+4*rdx+16]

; after
       488B4510             mov      rax, gword ptr [rbp+10H]
       8B5518               mov      edx, dword ptr [rbp+18H]
       83C202               add      edx, 2
       3B5008               cmp      edx, dword ptr [rax+8]
       730F                 jae      SHORT G_M55886_IG04
       8BD2                 mov      edx, edx
       488D449010           lea      rax, bword ptr [rax+4*rdx+16]

It's possible to construct contrived examples where the upper 32 bits are not zero: a[(int)checked(longVar + 2)]:

; before
       488B4510             mov      rax, gword ptr [rbp+10H]
       BA02000000           mov      edx, 2
       4863D2               movsxd   rdx, edx
       48035518             add      rdx, qword ptr [rbp+18H]
       7013                 jo       SHORT G_M55886_IG04
; index used as 64 bit, basically the cast to `int` was dropped
       483B5008             cmp      rdx, qword ptr [rax+8]
       7312                 jae      SHORT G_M55886_IG05
       488D449010           lea      rax, bword ptr [rax+4*rdx+16]

; after
       488B4510             mov      rax, gword ptr [rbp+10H]
       BA02000000           mov      edx, 2
       4863D2               movsxd   rdx, edx
       48035518             add      rdx, qword ptr [rbp+18H]
       7014                 jo       SHORT G_M55886_IG04
; 32 bit compare
       3B5008               cmp      edx, dword ptr [rax+8]
       7314                 jae      SHORT G_M55886_IG05
; zero extend to 64 bit
       8BD2                 mov      edx, edx
       488D449010           lea      rax, bword ptr [rax+4*rdx+16]