Remove unreachable error branch in memset calls (and in repeat) (#55985)

Some places use the pattern memset(A, v, length(A)), which requires a
conversion UInt(length(A)). This is technically fallible, but can't
actually fail when A is a Memory or Array.
Remove the dead error branch by casting to UInt instead.

Similarly, in repeat(x, r), r is first checked to be nonnegative, then
converted to UInt, then used in multiple calls where it is converted to
UInt each time. Here, only do it once.

base/array.jl

@@ -415,7 +415,7 @@ function fill!(a::Union{Array{UInt8}, Array{Int8}}, x::Integer)
     ref = a.ref
     t = @_gc_preserve_begin ref
     p = unsafe_convert(Ptr{Cvoid}, ref)
-    memset(p, x isa eltype(a) ? x : convert(eltype(a), x), length(a))
+    memset(p, x isa eltype(a) ? x : convert(eltype(a), x), length(a) % UInt)
     @_gc_preserve_end t
     return a
 end

base/genericmemory.jl

@@ -190,7 +190,7 @@ function fill!(a::Union{Memory{UInt8}, Memory{Int8}}, x::Integer)
     t = @_gc_preserve_begin a
     p = unsafe_convert(Ptr{Cvoid}, a)
     T = eltype(a)
-    memset(p, x isa T ? x : convert(T, x), length(a))
+    memset(p, x isa T ? x : convert(T, x), length(a) % UInt)
     @_gc_preserve_end t
     return a
 end

base/iddict.jl

@@ -126,7 +126,7 @@ function empty!(d::IdDict)
     d.ht = Memory{Any}(undef, 32)
     ht = d.ht
     t = @_gc_preserve_begin ht
-    memset(unsafe_convert(Ptr{Cvoid}, ht), 0, sizeof(ht))
+    memset(unsafe_convert(Ptr{Cvoid}, ht), 0, sizeof(ht) % UInt)
     @_gc_preserve_end t
     d.ndel = 0
     d.count = 0

base/strings/string.jl

@@ -570,9 +570,10 @@ julia> repeat('A', 3)
 ```
 """
 function repeat(c::AbstractChar, r::Integer)
+    r < 0 && throw(ArgumentError("can't repeat a character $r times"))
+    r = UInt(r)::UInt
     c = Char(c)::Char
     r == 0 && return ""
-    r < 0 && throw(ArgumentError("can't repeat a character $r times"))
     u = bswap(reinterpret(UInt32, c))
     n = 4 - (leading_zeros(u | 0xff) >> 3)
     s = _string_n(n*r)

base/strings/substring.jl

@@ -272,6 +272,7 @@ end
 
 function repeat(s::Union{String, SubString{String}}, r::Integer)
     r < 0 && throw(ArgumentError("can't repeat a string $r times"))
+    r = UInt(r)::UInt
     r == 0 && return ""
     r == 1 && return String(s)
     n = sizeof(s)