Split and simplify fgMorphOneAsgBlockOp

[SingleAccretion]

The main goal of this change is to extract the parts of fgMorphOneAsgBlockOp that actually do useful work and delete the rest.

To that end:

1. Import initobj <primitive> as stind <primitive zero>.
2. Fix some regressions and gather some improvements by retyping certain FP stores to in-memory locals to integer ones. This is where the bulk of the diffs for this change come from. Regressions are due to alignment or the fact some code gets a bit larger (but avoids a load from the data section).
3. Delete code from fgMorphOneAsgBlockOp: extract the "init block" part to the general block morpher, leave the copy block part as-is (for now).

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Issue Details

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The main goal of this change is to extract the parts of fgMorphOneAsgBlockOp that actually do useful and work and delete the rest.

To that end:

1. Import initobj <primitive> as stind <primitive zero>.
2. Fix some regressions and gather some improvements by retyping certain FP stores to in-memory locals to integer ones. This is where the bulk of the diffs for this change come from. Regressions are due to alignment or the fact some code gets a bit larger (but avoids a load from the data section).
3. Delete code from fgMorphOneAsgBlockOp: extract the "init block" part to the general block morpher, leave the copy block part as-is (for now).

Author:	SingleAccretion
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[SingleAccretion]

@dotnet/jit-contrib

The CI is down, but this is otherwise ready for review.

[SingleAccretion]

There are some SIMD-related failures here getting rid of which is best done by waiting for #76745 to land.

#76745 landed.

[SingleAccretion]

Diffs.

@dotnet/jit-contrib

[SingleAccretion]

The original retyping was done under MinOpts too, so this follows; there is of course an argument to be made it should not.

[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress, Fuzzlyn

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[SingleAccretion]

Failures are known:

1. [jitstress] Assertion failed 'isContainable || supportsRegOptional' in 'JIT.HardwareIntrinsics.General.Program:AbsDouble()' during 'Generate code' #78023 (stress)
2. Test failure System.Transactions.Tests.OleTxTests.ImplicitDistributedTransactions_cannot_be_changed_after_being_read_as_true #77241 (libraries stress)