The wasm backend didn't properly make use of all Cmm global registers
due to #24347. Now that it is fixed, this patch re-enables full
register mapping for wasm32, and we can now generate smaller & faster
wasm modules that doesn't always spill arguments onto the stack. Fixes #22460 #24152.

(cherry picked from commit 0cda2b8b)
(cherry picked from commit f1f5068b398b1effb837add38ecc5303dc9a381f)
(cherry picked from commit 1e695750)