I'm surprised there are no UTF-8 specific decode instructions yet, the way ARM has "FJCVTZS - Floating-point Javascript Convert to Signed fixed-point, rounding toward Zero"
FJCVTZS isn't really as specific to Javascript as the name suggests, it actually copies the semantics of an x86 instruction, which JS took its semantics from.
JS runtimes do use it but it's useful anywhere you need to do what x86 does, which obviously includes running x86 binaries under emulation.