Folding MInt64ToBigInt in MNot and MCompare will allow to use Int64 codegen during lowering.

For example:

let x = i64[0];
if (x > 100) { ... }

will then be compiled to:

[Codegen]                                 # LIR=LoadUnboxedInt64
[Codegen] movq       0x0(%rcx), %rax
[Codegen]                                 # LIR=CompareI64AndBranch:Gt
[Codegen] cmpq       $0x64, %rax
[Codegen] jle        .Lfrom84

Removing the conversion allows to generate LTestI64AndBranch instructions
for code like:

let x = i64TypedArray[i];
if (x) { ... }

This will now generate:

movq       0x0(%rcx), %rax
testq      %rax, %rax
je         .Lfrom00

This will allow to generate Int64 comparisons for elements loaded from
BigInt64 typed arrays.

For example:

let x = i64TypedArray[i];
if (x > 100) { ... }

Is now compiled to:

movq       0x0(%rcx), %rax
cmpq       $0x64, %rax
jle        .Lfrom00

https://hg.mozilla.org/mozilla-central/rev/9040c09caecd
https://hg.mozilla.org/mozilla-central/rev/7d0c74e617ec