Properly convert unsigned literals.
Forbid using unary + on a kAsmInt.
Forbid multiplies other than * 1.0 as being equivalent to unary +.

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=4203
TEST=test-asm-validator,asm-wasm
R=titzer@chromium.org,aseemgarg@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1832603002

Cr-Commit-Position: refs/heads/master@{#35046}