Rollup merge of #137028 - Zalathar:thir-pat-expr, r=compiler-errors

mir_build: Clarify some code for lowering `hir::PatExpr` to THIR

A few loosely-related improvements to the code that lowers certain parts of HIR patterns to THIR.

I was originally deferring this until after #136529, but that PR probably won't happen, whereas these changes should hopefully be uncontroversial.

r? Nadrieril or reroll

Author: jhpratt