[LoongArch] Align functions and loops better according to uarch

[LoongArch] Align functions and loops better according to uarch

The LA464 micro-architecture is very sensitive to alignment of hot code,
with performance variation of up to ~12% in the go1 benchmark suite of
the Go language (as observed by me during my work on the Go loong64
port).
[[ https://go.dev/cl/479816 | Manual alignment of certain loops ]] and [[ https://go.dev/cl/479817 | automatic alignment of loop heads ]]
helps a lot there, by reducing much of the random variation and
generally increasing performance, so we naturally want to do the same
here.

Practically, LA464 is the only LoongArch micro-architecture in wide use,
and we are currently supporting just that. The first "4" in "LA464"
stands for "4-issue", in particular its instruction fetch and decode
stages are 4-wide; so functions and branch targets should be preferably
aligned to at least 16 bytes for best throughput.

The Loongson team has benchmarked various combinations of function,
loop, and branch target alignments with GCC.
[[ https://gcc.gnu.org/pipermail/gcc-patches/2023-May/619980.html | The results ]]
show that "16-byte label alignment together with 32-byte function
alignment gives best results in terms of SPEC score". A "label" in GCC
means a branch target; while we don't currently align branch targets,
we do align loops, so in this patch we default to 32-byte function
alignment and 16-byte loop alignment.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D148622