Auto merge of #37445 - nnethercote:shrink-Expr_, r=eddyb

Shrink Expr_::ExprInlineAsm.

On 64-bit this reduces the size of `Expr_` from 144 to 64 bytes, and
reduces the size of `Expr` from 176 to 96 bytes.

For the workload in #36799 this reduces the RSS for the "lowering ast -> hir" phase and all subsequent phases by 50 MiB, which reduces the peak RSS for that workload by about 1%. Not huge, but it's a very easy improvement.

r? @eddyb

diff --cc src/librustc/hir/lowering.rs
Simple merge

diff --cc src/librustc/hir/mod.rs
Simple merge