cmd/compile/internal/ssa: allow memcombine on proved aligned addresses

[lxq015]

Currently, memcombine is disabled for architectures where unaligned
access might be slow or unsupported (unalignedOK = false), such as riscv64.
This leads to significant performance degradation for common operations
like encoding/binary.Uint64, which expand into long sequences of
byte loads, shifts, and ORs.

This change relaxes the global unalignedOK constraint in memcombine.
Instead of a total bailout, it now allows merging loads and stores
if the combined address can be statically proven to be naturally aligned.

By analyzing OpAddr, OpLocalAddr, and constant offsets through OpOffPtr,
the compiler can safely emit a single wide load (e.g., LD on RISC-V)
even when the architecture generally forbids unaligned access.
This provides the performance benefits of load merging for well-aligned
data structures without risking expensive kernel traps on unaligned-unfriendly
hardware.

Updates #736940

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[gopherbot]

Message from Jorropo:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Keith Randall:

Patch Set 1:

(1 comment)

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