[WebAssembly] Implement prototype f16x8.splat instruction. (#93228)

Adds a builtin and intrinsic for the f16x8.splat instruction.

Specified at:

https://github.com/WebAssembly/half-precision/blob/29a9b9462c9285d4ccc1a5dc39214ddfd1892658/proposals/half-precision/Overview.md

Note: the current spec has f16x8.splat as opcode 0x123, but this is
incorrect and will be changed to 0x120 soon.