[SLP] Provide an universal interface for FixedVectorType::get. NFC. (#96845)

SLP vectorizes scalar type to vector type. In the future, we will try to
make SLP vectorizes vector type to vector type. We add a getWidenedType
as a helper function. For example, SLP will make the following code

%v0 = load i32, ptr %in0, align 4
%v1 = load i32, ptr %in1, align 4
%v2 = load i32, ptr %in2, align 4
%v3 = load i32, ptr %in3, align 4

into a load <4 x i32>. The ScalarTy is i32 and VF is 4. In the future,
SLP will make the following code

%v0 = load <4 x i32>, ptr %in0, align 4
%v1 = load <4 x i32>, ptr %in1, align 4
%v2 = load <4 x i32>, ptr %in2, align 4
%v3 = load <4 x i32>, ptr %in3, align 4

into a load <16 x i32>. The ScalarTy is <4 x i32> and VF is 4.

reference:
https://discourse.llvm.org/t/rfc-make-slp-vectorizer-revectorize-vector-instructions/79436

Author: HanKuanChen