[clang-cl] map `/vlen=` (used for AVX512 and AVX10.1) to `-mprefer-vector-width=`

[zufuliu]

/vlen is a new option added in Visual Sudio 17.13, documented at https://learn.microsoft.com/en-us/cpp/build/reference/vlen?view=msvc-170

Arguments

/vlen=256
Specify a vector length of 256 bits for auto-vectorization and other optimizations.

/vlen=512
Specify a vector length of 512 bits for auto-vectorization and other optimizations.

/vlen
Specify the default vector length for the selected /arch setting.

Remarks

If a specific /vlen value isn't specified, the default vector length depends on the /arch flag setting. The /vlen flag can override the default vector length specified by /arch:AVX512 or /arch:AVX10.1 flag. For example:

• /arch:AVX512 /vlen=256 overrides the default vector length of 512 bits specified by /arch:AVX512 to be 256 bits.
• /arch:AVX10.1 /vlen=512 overrides the default vector length of 256 bits specified by /arch:AVX10.1 to be 512 bits.

When the specified /vlen value is incompatible with specified /arch flag, a warning is generated and default vector length for the /arch setting is used. For example:

• /arch:AVX2 /vlen=512 generates a warning because AVX2 doesn't support 512-bit vectors. Vector length of 256 bits is used in this case.