checker: type narrowing in none removes mut

[Le0Developer]

V version: V 0.5.0 75fc8be, press to see full `v doctor` output

V full version	V 0.5.0 75fc8be
OS	macos, macOS, 26.2, 25C56
Processor	10 cpus, 64bit, little endian, Apple M4
Memory	0.23GB/16GB
V executable	/Users/leodev/p/v/v/v
V last modified time	2026-01-31 10:45:14
V home dir	OK, value: /Users/leodev/p/v/v
VMODULES	OK, value: /Users/leodev/.vmodules
VTMP	OK, value: /tmp/v_501
Current working dir	OK, value: /Users/leodev/p/v/test
Git version	git version 2.52.0
V git status	weekly.2026.05-80-g75fc8bed
.git/config present	true
cc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
gcc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
clang version	Apple clang version 17.0.0 (clang-1700.0.13.3)
tcc version	tcc version 0.9.28rc 2026-01-10 HEAD@5ec0e6f8 (AArch64 Darwin)
tcc git status	thirdparty-macos-arm64 f995efa3
emcc version	N/A
glibc version	N/A

What did you do?
./v -g -o vdbg cmd/v && ./vdbg repro.v && repro

fn main() {
	mut x := ?int(none)

	if x == none {
		x = 10
	} else {
		x = 5
	}
}

What did you see?

repro.v:7:3: error: `x` is immutable, declare it with `mut` to make it mutable
    5 |         x = 10
    6 |     } else {
    7 |         x = 5
      |         ^
    8 |     }
    9 | }

What did you expect to see?

x is mutable, code should be compiled

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