Auto merge of #109843 - scottmcm:better-transmute, r=WaffleLapkin

Allow `transmute`s to produce `OperandValue`s instead of needing `alloca`s

LLVM can usually optimize these away, but especially for things like transmutes of newtypes it's silly to generate the `alloc`+`store`+`load` at all when it's actually a nop at LLVM level.

Author: bors