[AtomicExpand] Add bitcasts when expanding store atomic vector
AtomicExpand fails for aligned \`store atomic <n x T>\` because it
does not find a compatible library call. This change adds appropriate
ptrtoint + bitcast so that the call can be lowered, mirroring the
load-side handling from #148900.
[X86] Cast atomic vectors in IR to support floats
Extend the X86 \`alignedstore\` PatFrag to also match \`atomic_store\`
with vector-size alignment, so existing MOVAPS/MOVAPD/MOVDQA-family
aligned-store patterns cover 128-bit aligned vector atomic stores on
SSE/AVX/AVX-512 without per-type duplicates. \`<4 x float>\`,
\`<2 x double>\`, \`<2 x i64>\`, \`<4 x i32>\`, \`<8 x half>\`, \`<8 x bfloat>\`
all codegen to a single \`movaps\`/\`movapd\` on AVX+ via this.
Adds v8f16/v8bf16 bitconvert variants to the widen-path
\`atomic_store_32\` / \`atomic_store_64\` patterns so \`<2 x half>\`,
\`<2 x bfloat>\`, \`<4 x half>\`, \`<4 x bfloat>\` atomic stores reaching
the PR4 widen path also collapse to a single instruction on AVX+
targets.
Vectors whose \`getTypeAction\` is split rather than widen still rely
on PR6's \`SplitVecOp_ATOMIC_STORE\` — that path bitcasts the vector
to a scalar integer and issues an integer \`atomic_store_N\`, picked
up by the pre-existing scalar atomic-store patterns. The two
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[SelectionDAG] Split vector types for atomic store
Vector types that aren't widened are split so that a single ATOMIC_STORE
is issued for the entire vector at once. This enables SelectionDAG to
translate vectors with type bfloat,half.
[X86] Remove extra MOV after widening atomic store
This change adds patterns to optimize out an extra MOV present after
widening the atomic store. Covers <2 x i8> (SSE4.1+), <2 x i16>,
<4 x i8>, <2 x i32>, <2 x float>, <4 x i16>, <2 x ptr addrspace(270)>.
[SelectionDAG] Widen <2 x T> vector types for atomic store
Vector types of 2 elements must be widened. This change does this
for vector types of atomic store in SelectionDAG so that it can
translate aligned vectors of >1 size.
