Add replace_match_with_if_let assist

[Veykril]

Basically the counterpart to replace_if_let_with_match, I personally sometimes want to replace matches like

match foo {
    pat => expr,
    _ => (),
}

into the corresponding

if let pat = foo {
    expr
}

which is the main reasoning behind this.
I put this into the same file as replace_if_let_with_match because the are complementing each other and I would probably rename the file to something like replace_if_let_match but I didn't do that for now because I was unsure whether git would still view this as a rename or not due to the amount of changes in the file so that the diff is still properly visible for now.

[matklad]

Yesssss!

I've been wanting this for aeons!

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• TypeScript (windows-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)